Relationship between the habitat and diet of three species of atherinids and three species of gobies in a temperate Australian estuary

The distributions and diets of the six most abundant species of teleost in the shallows of a large south-western Australian estuary were examined from samples collected between March 1988 and February 1989. Fish were collected monthly by seine net from over bare sand and from within patchy and dense areas of the aquatic macrophyte Ruppia megacarpa. Their gut contents were compared with samples of the benthos and plankton collected from each of these three habitat types. The densities of the atherinids Leptatherina wallacei and Atherinosoma elongata and of the goby Favonigobius suppositus were greatest in dense R. megacarpa, whereas those of the atherinid L. presbyteroides and the goby F. lateralis were greatest over bare sand. The density of the goby Pseudogobius olorum was greater in patchy R. megacarpa than in the other two habitat types. The gut contents of each of the six species was dominated by crustaceans and/or polychaetes, with detritus also making a major contribution to the diet of P. olorum and F. lateralis. The relative proportions of prey items in the guts of fish in a particular habitat corresponded to the preys' relative occurrence in the environment. This indicates that the fish had been feeding predominantly in one particular habitat prior to capture. Within each habitat type, the six species partitioned the available food, the major components of the diet of each species being different. The gobiid species fed mainly on the benthos and the atherinids typically fed higher in the water column; A. elongata and P. olorum tended to be less selective as to where they foraged. There were no consistent differences in either the dietary breadth or the fullness of the guts of any species among habitat types.     

Seasonal and spatial changes in the larval fish fauna within a large temperate Australian estuary

A total of 66814 fish larvae, representing 37 families and 74 species, were collected in samples taken monthly between January 1986 and April 1987 from 13 sites located at frequent intervals throughout the large Swan Estuary in south-western Australia. The Gobiidae was the most abundant family, comprising 88.2% of the total number of larvae, followed by the Clupeidae (3.4%), Engraulididae (2.9%) and Blenniidae (1.0%). The most abundant species were Pseudogobius olorum (53.3%), Arenigobius bifrenatus (31.2%) and Engraulis australis (2.9%). Abundance of fish larvae in the lower, middle and upper regions of the estuary each reached a maximum between mid-spring and early summer, 2 to 4 mo before the attainment of maximum temperatures. Larvae of species such as Nematalosa vlaminghi and Apogon rueppellii were collected only between November and February, whereas those of others such as P. olorum, E. australis and Leptatherina wallacei were present over many months. The times and locations of capture of larvae have been related to the distribution and breeding periods of the adults of these species. The mean monthly number of species was far greater in the lower than upper estuary (14.7 vs 2.7), whereas the reverse was true for mean monthly concentration (42 vs 197 larvae per 100 m³). Classification, using the abundance of each of the 74 species recorded at the different sites, showed that the composition of the larval fish fauna in the lower, middle and upper estuary differed markedly from each other. Most larvae caught in the lower estuary belonged to marine species, whereas those in the upper estuary almost exclusively represented species that spawn within the estuary. The fact that the larvae of the 59 species of marine teleosts recorded during this study were restricted mainly to the lower estuary, and yet contributed only 6.2% to the total numbers for the whole estuary, helps to account for the relatively high species diversity in this region. The lack of penetration of many of these larvae beyond the first 12.5 km of the estuary presumably reflects the weak tidal effect in the wide basins of the middle estuary and saline regions of the tributary rivers. The larvae of the 13 teleosts that typically spawn within the estuary contributed 93.8% to the total numbers of larvae. Most of these estuarine-spawned larvae belong to teleosts that deposit demersal eggs and/or exhibit parental care (egg-guarding and oral and pouch-brooding), characteristics which would maximize their chances of retention within the estuary.     

Biannual spawning periods and resultant divergent patterns of growth in the estuarine goby Pseudogobius olorum: temperature-induced?

Samples of juveniles and adults of the goby Pseudogobius olorum were collected from seven sites in the shallows of the upper Swan Estuary, Western Australia, using a 3 mm-mesh seine net on one or two occasions in each month between September 1983 and April 1985. The mean gonadosomatic index of female fish rose from very low values in winter (June–August) to a sharp peak in mid-spring (October), reflecting the rapid maturation of ovaries over, this period. Ovaries with post-ovulatory follicles and ovaries that were undergoing degeneration were present, in November and December, but were then either rare or absent in those members of the corresponding cohort which survived into January and February. Female fish with advanced oocytes and mature ovaries were not found in December and January, but were present in February to April. The above trends exhibited by ovarian maturity indices, together with the appearance of larvae and small fish in both spring and autumn, demonstrate that P. olorum spawns in both spring and autumn and at best to only a limited extent in summer. Length-frequency and gonadal data show that the progeny of the spring-spawning group frequently spawn in the following autumn, when they are 5 mo old, and that those of the autumn-spawning group frequently spawn in the following spring, when they are 7 mo old. Some representatives of these two spawning groups survive through the winter and summer, respectively, to breed in a second season. Growth of the progeny of the spring-spawning group was relatively rapid between late spring and mid-autumn, whereas that of the autumn-spawning group was negligible during winter, but then inceased markedly in spring. It is proposed that the biannual spawning periods in each year by P. olorum in the Swan Estuary developed as a result of a rise in water temperature over the last few thousand years. Such a rise would have brought forward further into spring and extended later into autumn the periods when the water temperatures lie within the range (20 to 25°C) at which P. olorum typically spawns. However, mid-summer is now characterised by water temperatures >25°C, which are considered less conducive to reproductive success.     

Influence of site,season and year on contributions made by marine,estuarine, diadromous and freshwater species to the fish fauna of a temperate Australian estuary

Catches obtained at regular intervals by beach seining, gill netting and otter trawling at ten, four and six sites, respectively, have been used to determine the contribution of the different species and life-cycle categories of fish to the ichthyofauna of the large Swan Estuary in temperate south-western Australia between February 1977 and December 1981. These data were also examined to investigate the influence of site, season and year on the densities of the more abundant species. A total of 630 803 fish, representing 36 families and 71 species, were caught in the shallows using beach seines during this 5 yr study. Although the majority of these species were marine teleosts that were caught infrequently (marine stragglers), representatives of 7 of the 15 most abundant species were marine teleosts which entered the estuary regularly, and in large numbers (marine estuarine-opportunists). Of the remaining 8 most abundant species in the shallows, 7 completed their life cycle within the estuary (estuarine species) and 1 (Nematalosa vlaminghi) was anadromous, feeding for a period at sea and spawning in the upper reaches of the estuary. The contribution of individuals of the marine estuarine-opportunist category to catches in the shallows declined from nearly 95% in the lower estuary, to 17% in the middle estuary and 6% in the upper estuary. The estuarine and anadromous groups made a considerable contribution to the catches in both the middle and upper estuaries. By contrast, the contribution of freshwater species was small and even in the upper estuary accounted for only 0.2% of the catch. Site within the estuary generally influenced the catches of individual species to a greater extent than either season or year, or the interactions between these factors. When seasonal effects were strong, they could be related to summer spawning migrations into the upper estuary (Nematalosa vlaminghi, Amniataba caudavittatus), spring immigrations into the lower estuary (Mugil cephalus), or winter movements into deeper and more saline waters (Apogon rueppellii). Annual variations in the density of Torquigener pleurogramma were related to marked annual differences in the recruitment of the 0+age class.     

Factors influencing community structure and distribution of different life-cycle categories of fishes in shallow waters of a large Australian estuary

Fish were collected at regular intervals over 5 yr (February 1977 to December 1981) from ten shallow-water sites located throughout the lower, middle and upper regions of the large Swan Estuary in temperate southwestern Australia. Analysis of the catch data showed that the total number of species and total density of fishes were both influenced to a greater extent by site and season within the estuary than by year. The number of species and density of fishes within the whole system were greatest during the summer and autumn, when salinities and temperatures were at a maximum, and declined with distance from the estuary mouth. This reflects the trends shown by marine species, which comprise many species that occur only occasionally in the estuary (marine stragglers) and others which enter estuaries regularly and in considerable numbers (marine estuarine-opportunists). The density of marine estuarine-opportunists was also correlated with temperature, reflecting the tendency for the species of this category to congregate in the shallows during the summer and autumn. The similar seasonal aggregations of the single anadromous species and representatives of species that complete their whole life cycle in the estuary were frequently related to spawning. The density of the estuarine category was correlated neither with salinity nor distance from the estuary mouth. The number of estuarine species was also not correlated with distance from the estuary mouth. The density of freshwater species was inversely correlated with salinity and positively correlated with distance from the estuary mouth. The composition of the fish fauna changed progressively through the estuary, with that of the lower estuary being the most discrete. The composition also changed seasonally, particularly in the upper estuary where, during the winter and spring, the volume of freshwater discharge increased greatly and as a consequence the salinity declined markedly. The species diagnostic of the lower estuary were generally marine estuarine-opportunists, whereas those of the upper estuary typically belonged to either the estuarine or anadromous categories. The marine estuarine-opportunistMugil cephalus was, however, also one of the diagnostic species in the upper estuary during the winter and spring.Please address all correspondence and requests for reprints to I. C. Potter at Murdoch University     

Ichthyofaunas of a temperate estuary and adjacent marine embayment. Implications regarding choice of nursery area and influence of environmental changes

A fine-mesh seine net was used at regular intervals to collect fishes from the entrance channel and basin of the Blackwood River Estuary (south-western Australia), from Deadwater Lagoon, which is joined to the entrance channel by a narrow and shallow water-course and thus constitutes part of this estuary, and from Flinders Bay into which the estuary discharges. Sampling was at six-weekly intervals between February and December 1994. The juveniles of some marine species, such as Pelates sexlineatus, Rhabdosargus sarba and Aldrichetta forsteri, were either found only in the estuary or were in far higher densities in the estuary than in Flinders Bay. In contrast, the juveniles of some other marine species, such as Sillago schomburgkii, were relatively abundant in both environments, while others such as S. bassensis, Pelsartia humeralis, Lesueurina platycephala and Spratelloides robustus were either far more abundant in Flinders Bay or entirely restricted to this marine embayment. The various marine species found in inshore waters thus apparently vary considerably in their “preference” for estuaries as nursery areas. Although some marine species were abundant in the shallows of the estuary, the fish fauna of these waters was dominated by the estuarine-spawning species Leptatherina wallacei, Favonigobius lateralis, L. presbyteroides and Atherinosoma elongata. The above regional differences help account␣for the very marked difference that was found between the compositions of the shallow-water␣ichthyofaunas of Flinders Bay and each of the three estuarine regions. The ichthyofaunal compositions of the basin and channel underwent pronounced changes during winter, when freshwater discharge increased markedly and salinities in the estuary thus declined precipitously. This faunal change was mainly attributable to the emigration of marine stragglers, a reduction in the densities of marine estuarine-opportunist species such as Pelates sexlineatus and R. sarba, and the immigration of large numbers of both young 0+ Aldrichetti forsteri from the sea and of L. wallacei from the river. Although most of the above species were also abundant in Deadwater Lagoon, the ichthyofaunal composition of this region did not undergo the same seasonal changes, presumably due to the lack of riverine input and thus the maintenance of relatively high salinities throughout the year. The number of marine straggler species was much lower in Deadwater Lagoon than in the estuary basin, reflecting a far more restricted tidal exchange with the entrance channel. However, the overall density of fishes was far higher in Deadwater Lagoon than in the estuary basin or entrance channel, due mainly to the far higher densities of the estuarine species Atherinosoma elongata and L. wallacei and of the 0+ age class of the marine species R. sarba. The high densities of certain species in Deadwater Lagoon are assumed to be related, at least in part, to the high level of productivity and protection that is provided by the presence of patches of Ruppia megacarpa, an aquatic angiosperm that was not present in the estuary basin or entrance channel. Received: 3 December 1996 / Accepted: 19 December 1996     

The fish fauna of a seasonally closed Australian estuary. Is the prevalence of estuarine-spawning species high?

The fish at sites located throughout the large, seasonally closed Wilson Inlet, on the southern coast of Western Australia, were sampled bimonthly between September 1987 and April 1989. Seine nets were used to sample nearshore shallow waters, while gill nets were employed in slightly more offshore and deeper waters. Twenty species were recorded in the shallows, of which the three species of atherinid and the three species of goby comprised >97% of the total catch. In terms of number of individuals, the 27 species recorded in gill nets in the deeper waters were dominated by Cnidoglanis macrocephalus and Platycephalus speculator, and to a lesser extent Engraulis australis, Aldrichetta forsteri, Sillaginodes punctata and Arripis georgianus. Fifty-five percent of the species recorded in the nearshore shallow waters and 18% of those in offshore deeper waters spawn within Wilson Inlet; these species contributed 98.5 and 63.0%, respectively, to the total catches in those waters. Classification and ordination showed that the composition of the fauna in the shallows was similar at all sites throughout the large basin and did not change conspicuously with season. However, the composition of catches taken in offshore waters differed between the lower part of the basin and the middle and upper regions of the basin, which in turn differed from those in the saline reaches of a tributary river. The four diagnostic species of the lower estuary were all marine species, while the three diagnostic species in the river included a marine species (Mugil cephalus) that often penetrates far upstream in other systems, and a species which was confined to the rivers (Acanthopagrus butcheri). The composition of the fish fauna in the offshore waters of the lower estuary between the middle of spring and middle of autumn was different in 1987/1988 (when the estuary mouth was open for only the first two months of that period) from that in 1988/1989 (when the mouth was open for the whole of that period). This difference is related to the greater number of marine species that were retained in the first of these years, when the estuary was open to the ocean for only a short period. The greater retention of marine species in 1987/1988 than in 1988/1989 probably reflects a far lower level of freshwater flusing and/or a less marked decline in salinity.     

Influence of environmental variables on the fish fauna of the deeper waters of a large Australian estuary

Fish were collected by gill nets from the deeper waters of the Entrance Channel, basins and rivers of the large Peel-Harvey estuarine system (south-western Australia) in the wet (June to November) and dry (December to May) periods between August 1979 and July 1981. Simple-regression analysis showed that the number of species, abundance and biomass of fish in the rivers rose with increases in the salinity and temperature of both the surface and bottom of the water column. No such significant correlations were found in the Entrance Channel andbasins (Peel Inlet and Harvey Estuary), where salinity changes were far less marked. The number of species at sites throughout the estuary was inversely correlated with distance from the estuary mouth. Multiple-regression equations showed that, compared with the other environmental variables tested, bottom salinity had a greater influence on the nunber of species and abundance both in the rivers and in the system as a whole. These results indicate that salinity has a greater effect on the fauna in the deeper waters than in the shallows (cf. Loneragan et al., 1986). The larger fish which characterise the deeper waters may thus be less tolerant to low salinities than the smaller fish typically found in the shallows. Both classification and ordination separated the faunal composition of the rivers from those of the Entrance Channel and basins. The fauna of the two narrow and deeper sites in the rivers separated into wet- and dry-period components. Differences between the faunal composition of the riverine regions and those of the Entrance Channel and basins have been related to the much more variable and lower minimum salinities in the rivers. Species characteristic of the rivers included Amniataba caudavittatus, which is estuarine sensu stricto in south-western Australia, the semianadromous Nematalosa vlaminghi and the highly euryhaline Mugil cephalus. The indicator species for the Entrance Channel and basins were all marine species (Cnidoglanis macrocephalus, Hyporhamphus melanochir, Gerres subfasciatus and Pomatomus saltator).     

Classification and ordination of benthic samples from Bedeque Bay,an estuary in Prince Edward Island,Canada

An attempt was made to identify the causes of the distribution of benthos within Bedeque Bay using multivariate techniques programmed for the computer. Both classification by a hierarchical cluster analysis, and ordination by principal components analysis suggested that a large proportion of the variance in the data was directly or indirectly correlated with a salinity gradient. Classification divided the species into two main groups, a in the upper half of the estuary where lower salinities and larger salinity fluctuations occurred, and group b in the lower half of the estuary with a more stable salinity regime. The group b species were further subdivided into those preferring soft mud and those preferring sandier sediments. The group a species were divided into a well-developed oyster association and various sub-groups less strongly associated with oysters. Five principal components were required to account for 50% of the variance in the data. The first axis accounted for 20% of the variance and was shown by a non-parametric test to be correlated with the salinity gradient. Axes II to V could not be interpreted, but possibly represented complex species interactions. By providing hard substrates and altering the natura of the sediment, oysters and mussels produced conditions suitable for many other species.     

Effects of environmental factors on radiocadmium uptake by four species of marine bivalves

Temperature, salinity, bottom-sediment type, and zinc concentration all influenced Cd uptake by 4 marine bivalves (Mya arenaria, Mytilus edulis, Mulinia lateralis and Nucula proxima) in short-term static assay systems using ¹⁰⁹Cd as a tracer. The experimental system consisted of aquaria containing 20 l of seawater maintained under controlled light and temperature conditions. The water contained either 5 or 20 g/l Cd and tracer. Distribution and kinetics of the metal were monitored in the water column and organisms. The results demonstrate that Cd uptake rates differed widely among the organisms tested. An increase in temperature increased Cd uptake rate by all test organisms. A decrease in salinity increased Cd uptake by all organisms tested. The presence of bottom sediment depresses Cd accumulation in some benthic animals. Zinc in concentrations of 0.5 mg/l substantially decreased Cd uptake by Mytilus edulis and Mulinia lateralis. It is suggested that all important species and environmental variables be considered when studying heavy-metal uptake by marine organisms or when establishing water-quality criteria.     

The ichthyofaunal composition of the Elbe Estuary: an analysis in space and time

A stow net (90 m² opening) was used, at regular intervals between July 1989 and June 1993, to collect fish over ca. 4 h on both the flood and ebb tides at five sites distributed at regularly spaced distances throughout the Elbe Estuary. Overall, 53.4% of the species were marine, and some of these were abundant, e.g.Clupea harengus, Pleuronectes flesus, Pomatoschistus minutus, Merlangius merlangus and Sprattus sprattus. However, in terms of number of individuals, such species contributed only 9.1%, compared with 90.0% by anadromous species, which was predominantly due to exceptionally high catches of Osmerus eperlanus. The ichthyofaunal compositions on the flood and ebb tides were similar, even downstream where salinities change markedly during each tidal cycle. Although this is probably due in part to groups of fish being swept first upstream on the flood tide and then back downstream on the ebb tide, several species clearly tended to occupy particular regions of the estuary. Each year the species composition changed sequentially from the most downstream site (max. salinity=ca. 31.4‰) to the most upstream site (max. salinity=ca. 1.5‰), mainly as a result of a sequential decline in the number and abundance of marine species and a progressive increase in the contributions of both anadromous and freshwater species. However, the marine species diagnostic of differences in the compositions among regions varied between years. Thus, for example, the diagnostic species for the downstream sites were C. harengus and M. merlangus in 1990/1991 and S. sprattus in 1992/1993, reflecting differences in the recruitment strengths of these marine species in the two years. The ichthyofaunal composition exhibited pronounced annual cyclical changes in each region and thus occurred irrespective of whether or not the salinity in a region underwent pronounced changes during the year. This cyclicity was attributable to intra-annual variations in the times of recruitment of certain marine species, particularly in the downstream region, and to seasonal migrations of diadromous species into each region, as well as to the movements of freshwater species into the upstream region in winter and early spring. Received: 18 November 1999 / Accepted: 25 October 2000     

Active salinity choice and enhanced swimming endurance in 0 to 8-d-old larvae of diadromous gobies,including Sicydium punctatum (Pisces), in Dominica,West Indies

We studied the early life history of diadromous gobies in Dominica, West Indies, from May 1989 to May 1991, emphasising Sicydium punctatum Perugia. The transition of newly hatched larvae from upriver nest sites to the sea was studied in laboratory experiments. Newly hatched larvae are negatively buoyant but avoid settling to the bottom by active swimming during drift to the sea. Laboratory experiments evaluated salinity preferences and effects on swimming endurance. Larvae in haloclines actively selected low to intermediate salinities. Initially (0 to 5-d post-hatch), larvae minimized exposure to salinities >10 ppt, but later (5 to 8-d) occupied increasingly saline water. Larvae in no-choice freshwater or seawater treatments ceased activity at 4 to 5 d, but in haloclines larvae remained active up to 8 d post-hatch. Salinities <10 ppt are important for early survival of sicydiine gobies. Implications for larval survival and transport are discussed.     

Protracted estuarine phase in the life cycle of the marine pufferfish Torquigener pleurogramma

The present study was undertaken to elucidate the way in which the Swan Estuary in south-western Australia is used by the common blowfish Torquigener pleurogramma, a representative of the abundant and widely distributed family Tetraodontidae. T. pleurogramm were collected by beach seine and otter trawl from the Swan Estuary between February 1977 and December 1980 and between May 1984 and February 1986. While T. pleurogramma feeds on a wide variety of organisms in the estuary, the main components of its diet are polychaetes and amphipods for fish <130 mm and bivalve molluscs for larger fish. Numbers of blowfish were inversely correlated with water depth, with densities on the banks (water depth <1.5 m) sometimes reaching 5 fish m^-2, and tended to be greater at night than during the day. The density of T. pleurogramma in the shallows was positively correlated with salinity and inversely correlated with distance from the estuary mouth. Numbers increased greatly in the latter half of 1980 and 1985 as a result of the recruitment of large numbers of the 0+ age class (i.e., fish in their first year of life). Blowfish were represented by seven age classes in the estuary and attained a maximum size of 230 mm (220 g). By the end of their first and second years of life, fish had reached approximately 90 mm (14 g) and 125 mm (39 g), respectively. Sexual maturity was generally not reached until the end of the second year of life. The presence of higher gonadosomatic indices and more mature gonads in fish collected just outside than within the estuary indicate that T. pleurogramma leaves the estuary before spawning. Comparisons between lengthfrequency data, allied with information on the prevalence and intensity of gill parasites, indicate that assemblages in estuarine and neighbouring inshore waters remain distinct for many months and that growth within the estuary is faster than in inshore marine environments.     

Transient ontogenetic expression of hermaphroditic gonad morphology within the Gobiosoma group of the Neotropical seven-spined gobies (Teleostei: Gobiidae)

The Neotropical seven-spined gobies (tribe Gobiosomatini), including the Gobiosoma and Microgobius groups, constitute a speciose, monophyletic gobiid taxon. In particular, member species of the Gobiosoma group exhibit a combination of behavioral diversification and microhabitat specialization that may have played a major role in their collective rapid evolution and high rate of speciation. Functional hermaphroditism, which is common among gobiids, can promote exploitation of patchily distributed micro-niches by countering potential reductions in reproductive opportunities. However, the possible role of functional hermaphroditism in the exploitation of microhabitats and rapid speciation in the Gobiosoma group is unknown. One member species, Tigrigobius multifasciatus, is a functional hermaphrodite. Two other Gobiosoma group species are functionally gonochoric (i.e., constant-sexed), but exhibit transient hermaphroditic ovarian structure among immatures. In this study, ovarian morphology among immature and adult females of several Gobiosoma group species was examined to see if hermaphroditic gonadal features are present. Although no evidence of functional hermaphroditism in the form of precursive accessory gonadal structures (pAGS) associated with the adult ovary was found among newly examined species, all species exhibited the transient expression of hermaphroditic gonadal features associated with the immature ovary. In contrast, among six species of non-Gobiosomatini genera having no record of hermaphroditism, none exhibited similar transient hermaphroditic features associated with the immature ovary. These findings suggest that hermaphroditism may have been an ancestral trait which has been secondarily lost within some Gobiosoma group species. This study also shows that ontogenetic and morphogenic processes within the gobiid reproductive system may provide new insights into the evolution of life history traits and significantly further our understanding of the extraordinary evolutionary success of this group.     

Physiological ecology of four Polysiphonia species (Rhodophyta,Ceramiales)

Photosynthesis and respiration of 4 species of the marine red algal genus Polysiphonia were evaluated under a variety of light, temperature and salinity conditions. The manometric results were compared with the local distribution and abundance of each species. The species can be separated into two distinct categories based on their overall distribution and temperature optima: (1) cold water plants [P. lanosa (L.) Tandy and P. elongata (Hudson) Sprengel], with peak photosynthesis at 21° to 24°C, but with active photosynthesis as low as 5°C; (2) plants with warm-water affinities [P. nigrescens (Hudson) Greville and P. subtilissima Montagne], having photosynthetic optima at 27° to 30°C, and exhibiting little or no photosynthesis below 10°C. The plants from the first group exhibit thermal injury at temperatures of 25°C and show a narrow tolerance to low salinities during periods of high temperatures. The plants from the second group show thermal injury at 30°C and have a wider tolerance to low salinities. The horizontal distribution of the 4 Polysiphonia species within the Great Bay Estuary System of New Hampshire, USA, is primarily governed by their tolerances to high temperatures and low salinities. The temperature optimum for each of the species corresponds to its particular estuarine distribution. Thus, P. subtilissima, having the highest temperature optimum, penetrated furthest into the Estuary, while P. lanosa, having the lowest temperature optimum, was restricted to the more coastal stations. There was a good correspondence between the natural distribution patterns and the manometric results.Published with the approval of the Director of the New Hampshire Agricultural Experiment Station as Scientific Contribution No. 731.Scientific Contribution No. 4 of the Jackson Estuarine Laboratory.     

A response-surface approach to the combined effects of temperature and salinity on the larval development of Adula californiensis (Pelecypoda: Mytilidae). I. Survival and growth of three and fifteen-day old larvae

Laboratory experiments of a factorial design were used to examine the combined effects of temperature and salinity on the survival and growth of early and late-stage larvae of Adula californiensis (Phillippi, 1847). Response-surface curves were generated to predict optimal conditions for survival and growth in order to better understand the successful recruitment of this species within the Yaquina Bay estuary (Oregon, USA). Three-day old cultured larvae were more sensitive to reduced salinity than were 15-day old larvae. However, the 15-day old larvae showed a narrower temperature tolerance than the 3-day old larvae. A. californiensis larvae survived over a wider range of temperatures near optimum salinities than at salinities near their lower tolerance limit, and conversely. Temperature and salinity ranges for maximum survival (10° to 15°C, 31 to 33) were narrower than the ranges which occur within the estuary where the adult populations exist. Larval size did not increase markedly during the 15-day rearing period, and was not greatly affected by temperature or salinity. No statistically significant temperature-salinity interaction was found for either survival or growth.     

Aspects of sodium regulation in a brackish-water and a marine species of the isopod genus Sphaeroma

Sodium and chloride concentrations of the haemolymph were determined in Sphaeroma rugicauda Leach and S. serratum Fabricius acclimatized to a range of salinities. Sodium loss and uptake (using ²²Na) were measured in salinities below 1.1 for S. rugicauda and 7.2 for S. serratum. Potential differences between haemolymph and medium indicate active uptake of both ions in certain salinities. The active-uptake component of total sodium-influx was found to be related non-linearly to the sodium concentration of the medium. Curves of were fitted to the activeuptake data. Km values indicate that, in S. rugicauda, the sodium uptake system has a greater affinity for sodium than in S. serratum. These findings are discussed in relation to the distribution of each species.     

Reproduction under predation risk in the sand goby,Pomatoschistus minutes,and the black goby,Gobius niger: the effect of age and longevity

Summary In aquarium experiments, the two marine gobiid fish species Pomatoschistus minutus and Gobius niger were allowed to build nests and to spawn in the presence and absence of a predator (cod, Gadus morhua); behind a glass wall the predator was kept where it could be clearly seen by the gobies and vice versa. P. minutus showed no difference in number of nests or number of spawnings in the different treatments; approximately half of the males built nests, and the females spawned in half of those nests. G. niger, on the other hand, responded differently to the simulated predation risk. No nests were built in sight of the predator, whereas in the absence of predators, half of the males built nests and received eggs. The G. niger individuals in this experiment were 2–3 years old. However, when comparing the reproduction of G. niger of different age in the presence of a predator, older individuals (4–5 years) spawned, whereas younger (2–3 years) did not. No difference in vulnerability towards predators was found between equal-sized P. minutus and G. niger. The optimal behavior during the breeding season must depend on prospects of survival, based both on maximal lifespan and vulnerability to predation.     

Composition of the fish fauna of a permanently open estuary on the southern coast of Australia,and comparisons with a nearby seasonally closed estuary

The fish faunas of the outer basin (Nornalup Inlet), inner basin (Walpole Inlet) and saline region of the main tributary (Frankland River) of the permanently open Nornalup-Walpole Estuary on the southern coast of Western Australia, were sampled bimonthly for a year using seine and gill nets, and also during a further two months by the former method. Although the Nornalup-Walpole Estuary is permanently open, the catches of fish in its shallows were dominated (98.4%) by estuarine-spawning species, thereby paralleling the situation in the nearby and seasonally closed Wilson Inlet. In contrast, larger representatives of several marine species were present in appreciable numbers in the offshore, deeper waters of both of these estuaries. The delayed recruitment of marine species into these estuaries apparently reflects the distance that the juveniles of these species have to travel from the areas where they are believed predominantly to spawn. The larger representatives of marine species made a greater contribution to the fish faunas of the offshore, deeper waters in the Nornalup-Walpole Estuary than in Wilson Inlet (64.5 vs 36.9%) and, unlike the situation in the latter estuary, they included five species of elasmobranchs, two of which (Mustelus antarcticus and Myliobatis australis) were relatively abundant. Classification and ordination of the combined data for both estuaries demonstrated that the composition of the fish fauna in the offshore, deeper waters of the outer basin of the Nornalup-Walpole Estuary was particularly distinct, with some marine species being restricted to these waters. This is presumably related both to the presence of a permanently open entrance channel and the relatively deep waters found in Nornalup Inlet, which allow the ready exchange of water between the sea and estuary and the maintenance of high salinities in the deeper regions of the outer basin for much of the year. The fish faunas in Walpole Inlet and the tributaries of both the Nornalup-Walpole Estuary and Wilson Inlet were more similar to each other than they were to those in the more seawards end of either estuary. This similarity reflects the apparent preference of certain teleosts, such as the estuarine species Acanthopagrus butcheri and the marine species Mugil cephalus and Aldrichetta forsteri for reduced salinities and/or features associated with riverine environments.     

Acclimation responses to salinity of three estuarine red algae from New Jersey

The effects of salinity and acclimation time on the net photosynthetic responses of 3 estuarine red algae, Bostrychia radicans Mont., Caloglossa leprieurii (Mont.) J. Ag., and Polysiphonia subtilissima Mont., from Great Bay Estuary, New Jersey, USA, were investigated. The algae were cultured in a series of synthetic seawater media of 5, 15, 25 and 35% S for acclimation periods of 0, 2, 4, 8, and 16 days prior to determining their photosynthetic responses. All species were euryhaline, and demonstrated photosynthesis at all the above salinities. B. radicans, which was more common towards the mouth of the estuary, had a maximum photosynthetic rate at 25% S, whilst C. leprieurii and P. subtilissima, which were more common towards the head of the estuary, had photosynthetic maxima between 15 and 25%, and at 15%, respectively. The curves relating net photosynthesis to salinity were usually similar within a species at different acclimation periods, although statistically significant differences were sometimes noted. The acclimation periods producing maximal net photosynthesis were 0, 2 and 4 days for B. radicans, and 4 days for C. leprieurii, whilst for P. subtilissima there was no significant difference in response for any acclimation period over the range of salinities studied.