Ecological investigations of the late-stage phyllosoma and puerulus larvae of the western rock lobster Panulirus longipes cygnus

The distribution and abundance of the late-stage phyllosoma larvae of Panulirus longipes cygnus George and the distribution and densities of the final larval stage, the puerulus, both in the plankton and at settlement along the coast, were investigated. A total of 3,617 late-stage phyllosoma (Stages VI to IX) and 301 puerulus larvae were caught at 187 plankton stations during the July to November periods 1974, 1975 and 1976 off the west coast of Australia between 29°00 to 32°30S and 113°30 to 115°00E. The depth range sampled was 0 to 35 m on the continental shelf and 0 to 90 m off the shelf. During onshore/offshore cruises with similar sampling effort on and off the shelf, 1,169 late-stage phyllosoma larvae were taken, of which only 9 were caught on the shelf, and these near the outer edge. A series of cruises sampling two areas beyond the shelf near 29°30 and 32°00S yielded 2448 late-stage phyllosoma, with greater densities of larvae in the northern location. The settlement of puerulus-stage larvae along the coast in the same geographical range was also greater in the north than in the south. The data from the onshore/offshore cruises showed a definite effect of moon phase on numbers of puerulus larvae caught on the shelf, with higher catches near new moon. The low numbers of puerulus larvae (usually 0, 1 or 2 individuals) caught at all stations showed that the puerulus stage is sparsely distributed in the plankton. Fewer puerulus larvae were present at the surface than at lower depths, but it was not possible to determine a depth preference for the puerulus between 10 m and the lowest depths sampled because of the low catch numbers. No relationships were found between puerulus larvae density and surface-water temperature, salinity, or plankton biomass at each station. Data on the larval distributions indicate that, near the end of their planktonic existence, the majority of the late-stage phyllosoma larvae of P. longipes cygnus are not carried onto the shelf, where mixing of oceanic and continental shelf waters occurs only on the outer third, but are transported southward by oceanic circulation beyond the shelf. The puerulus moults from the last phyllosoma stage beyond the shelf and completes the larval cycle by swimming across the shelf and settling in the shallow reef areas.     

Spatial variation of growth and gonadal developments of Maurolicus muelleri in the Norwegian Sea and in a Norwegian fjord

The epibenthic megafauna of the high-Arctic Northeast Greenland shelf was investigated by means of seafloor photography and Agassiz trawl catches. At 54 stations in water depths between 40 and 770 m, sequences of color slides, each depicting about 1 m² of the seafloor, were obtained along photographic transects of about 100 to 600 m length. The photographs were quantitatively analyzed for abundance of epibenthic organisms identified by comparison with specimens collected from trawl catches. Megabenthic biomass was estimated by multiplying density values with averge body mass figures. For five dominant brittle star species, the population oxygen uptake and, thus, organic carbon mineralization potential were approximated by applying individual respiration rates of average-sized specimens to density figures. Multivariate analyses of the megabenthic species distribution revealed a distinct depth zonation. Shallow shelf banks (<150 m), characterized by coarse sediments, many stones and boulders as well as negative bottom water temperatures, housed a rich epifauna (30 to 340 ind m^–2, 1.8 to 10.5 g AFDW m^–2), strongly dominated (80 to 98% by numbers) by the brittle stars Ophiocten sericeum and Ophiura robusta. The oxygen uptake by brittle stars ranged from 0.4 to 95 mol O₂ m^–2 h^–1 (i.e., assuming a respiratory quotient of 0.8, an organic carbon mineralization of 0.1 to 21.9 mg C m^–2 d^–1). At the bank flanks sloping to the shelf troughs (100 to 580 m), finer sediments prevailed, stones were rare, and bottom water temperatures were positive due to the inflow of Atlantic water. Compared to bank sites, total epibenthic abundances as well as carbon mineralization by brittle stars were roughly ten times and total biomass about four times smaller. In deep shelf depressions as well as at the continental slope (200 to 770 m), stones were completely lacking, and sediments very fine. Epibenthic standing stock and carbon mineralization were one to two orders of magnitude lower than on the banks. The estimation of brittle star oxygen uptake indicates that a considerable portion of the organic carbon produced in the polynya and partitioned to the benthos may be remineralized by epibenthic bank assemblages.     

Distribution et synécologie des fonds à Gryphus vitreus (Brachiopoda) en Corse

In the Mediterranean Sea, the brachiopod Gryphus vitreus (Born) was collected off Corsica by dredging and trawling during three cruises (June 1983; April and July 1984) and from along the French continental coast by dredging during many cruises (1975–1985). The upper limit of its bathymetric distribution lies between 115 and 125 m (rarely, from 90 m), the lower limit between 140 and 200 m depth, forming a belt along the edge of the continental shelf. The horizontal distribution of G. vitreus varies from 250 m to >4 km, and is directly related to geomorphological and hydrodynamic variations resulting from local topography and/or geographical location. The G. vitreus bottoms are bounded on the continental slope by the bathyal mud biocoenosis and on the shelf by the coastal detritic or shelf-edge detritic biocoenosis, both circalittoral. The bottoms correspond to a bathylittoral ecosystem characterized by (1) prevailing environmental conditions which result in a biotope characterized by coarse detritic sediment clogged by fine sand and mud with a high content of small hard substrates, by constant moderate bottom-currents which become weaker with increasing depth, and by small annual variations in temperature and salinity; the occurrence of an ostreobium sp. alga in the shell of G. vitreus indicates the involvement of the phytal system, as defined by Pérès (1982); (2) the G. vitreus biocoenosis, comprised of characteristic macrobenthic stocks (particularly polychaetes, mollusks, and epifaunal species on small hard substrates). Whatever the geomorphology, neighbouring circalittoral and bathyal biocoenoses have little influence on the G. vitreus biocoenosis, which displays a comparatively greater species richness and density. However, immigration of circalittoral species is encouraged by detrital sediments and water currents. The occurrence of this ecosystem as a distinct belt intercalated between circalittoral and bathyal zones is discussed. The ecosystem is considered to be bathylittoral.     

Depth zonation and metabolic adaptation in Dover sole,Microstomus pacificus,and other deep-living flatfishes: factors that affect the sole

Flatfishes of Monterey Bay, central California, undergo species replacements with increasing depth along a transect from 100 m on the continental shelf down to a depth of 1400 m on the continental slope. The Dover sole, Microstomus pacificus, differs from the other local flatfish species by undergoing an extensive ontogenetic vertical migration, occupying all depth zones at different life stages, and having its maximum spawning biomass in the oxygen minimum zone between 600 and 1000 m. Size-activity relationships and depth-activity relationships for the glycolytic enzyme lactate dehydrogenase (LDH) and for two enzymes associated with aerobic metabolism, malate dehydrogenase and citrate synthase (CS), were examined in white-muscle tissue of shallow-living, deep-living and ontogenetically-migrating species. Scaling coefficients (b) for weight-specific enzyme activity (log activity)=a+b (log wet weight), varied in sign as well as magnitude for fishes living at different depths. In the shallow-living California halibut Paralichthys californicus, LDH scaled positively (0.39) and CS scaled negatively (-0.15) with size, a pattern observed previously for most shallow-water fish species. The permanently deep-living species, the deepsea sole Embassichthys bathybius, differed in that both LDH and CS scaled strongly negative (-2.0 and-1.5, respectively). For the ontogenetically migrating Dover sole Microstomus pacificus, there was a shelf-slope transition. For the shelf specimens (200 m), LDH scaled positive (0.11) and CS negative (-0.29) and for the slope specimens (400 m), LDH scaled negative (-0.65) and CS strongly negative (-0.63). Rex sole, Glyptocephalus zachirus, showed a similar shelf-slope transition. Intraspecific depth-enzyme activity differences were not incremental, but changed abruptly between the continental shelf stations (100 to 200 m) and the continental slope (400 to 1400 m). Based on comparisons with laboratory-maintained individuals, the decline in the metabolic capacity of the white muscle of Dover sole is a phenotypic response to the low food and oxygen conditions of the continental slope. Contrary to expectation, anaerobic capacity (LDH activity) decreased in response to low oxygen conditions, suggesting that in a permanently hypoxic environment such as the oxygen minimum zone the metabolic strategy may be to not incur an oxygen debt that would be difficult to pay back.     

Distribution of deep-water gorgonian corals in relation to benthic habitat features in the Northeast Channel (Atlantic Canada)

The distribution and abundance of deep-water gorgonian corals were investigated along 52 transects at 183–498 m depth in the Northeast Channel, between Georges Bank and Browns Bank in the northwest Atlantic, using a remotely operated vehicle and a towed video-camera system. Three species (Paragorgia arborea, Primnoa resedaeformis, and Acanthogorgia armata) were observed. Primnoa occurred on 35 transects below 196 m depth, with highest local abundance in stands of 104 colonies per 100 m². Paragorgia was present on 21 transects deeper than 235 m, with highest local abundance of 49 colonies per 100 m². Acanthogorgia was observed at only four transects between 231 m and 364 m, with a local maximum abundance of 199 colonies per 100 m². The maximum abundance averaged for whole transects was 19.2 and 6.2 colonies per 100 m² for Primnoa and Paragorgia, respectively. The corals were more common in the outer part of the channel along the shelf break and slope than on the shelf in the inner part. All three species showed a patchy distribution with no signs of competitive exclusion at any spatial scale. Transects with high abundance of corals were characterised by depths greater than 400 m, maximum temperatures less than 9.2°C, and a relatively high percentage coverage of cobble and boulder (more than 19% and 6%, respectively). High temperatures probably control the upper depth limit of the corals, and Primnoa seems to tolerate slightly higher temperatures than Paragorgia. Abundance of both species was negatively correlated with average temperature and positively with cobbles. Together, temperature, percentage cobble and salinity accounted for 38% of the variance of Primnoa. The comparable figure for Paragorgia was 15%. The observed distribution indicated that the abundance of coral is controlled by additional factors such as larger-scaled topographic features governing the current regimes and thus also the supply of food and larvae.Communicated by R. J. Thompson, St. Johns     

Non-viable seasonal settlement of larvae of the upper bathyal brittle star Ophiocten gracilis in the Rockall Trough abyssal

Re-examination of ine structure of postlarvae previously identified as the abyssal species Ophiura ljungmani (Lyman) in a time series of samples taken from 1975 to 1980 from a 2900 m deep permanent station in the southern Rockall Trough (Northeast Atlantic Ocean) has shown the majority of the large summertime peak of brittle-star postlarvae to belong to a second species. Most were incomplete and probably corpses when collected. The few intact specimens present were identical to postlarvae collected in benthic samples from the neighbouring continental slope, where a dense population of the bathyal species Ophiocten gracilis (G. O. Sars) is present from 600 to 1 200 m depth. Examination of population size-structures indicates synchronous reproduction in January/February and annual recruitment to the population. The presence both of Ophiopluteus ramosus and intact postlarvae of Ophiocten gracilis in midwater plankton samples from the vicinity of the permanent station in March/April are thought to derive either from the adjacent slope population or, less probably, from larvae transported in the North Atlantic Current from the Flemish Cap area off Newfoundland. In deep water, viability of these benthic postlarvae is low, none surviving longer than the following winter in the Rockall Trough. These observations suggest a large seasonal transfer from the slope of non-viable larvae and postlarvae as food to the abyssal benthic community.Communicated by J. Mauchline, Oban     

The role of diatom resting stages in the onset of the spring bloom in the East China Sea

The abundance and species composition of diatoms were investigated along the PN line from the Okinawa Islands to the inner continental shelf in the East China Sea in the early spring of 1996. Viable diatom resting stages in sediments on the shelf were also enumerated by the extinction dilution method (most probable number method). Clear differentiation in the water masses was observed, with less saline, cold water (shelf water) on the shelf region, and warm, saline water (Kuroshio water) prevalent off the shelf and on the shelf edge. In the Kuroshio water, the abundance of diatoms was generally low but species diversity of diatoms was relatively high. In contrast, the spring bloom of diatoms was clearly observed in the shelf water where the water column was weakly stratified. The bloom was dominated by Chaetoceros debilis, contributing occasionally >80% of the diatom community. Resting stages of this species were also the most abundant taxon in the sediments on the shelf. Resuspension of the sediment during winter mixing of the water column should have enabled the resting stages to germinate at the surface. Subsequent vegetative growth after germination led to the formation of an early spring bloom of C. debilis when the water column was stratified and light availability had increased. Intermittent resuspension of sediment on the shelf, driven by strong winds and tidal currents, probably provides opportunities for diatoms with a resting stage to exploit favorable conditions for their germination and subsequent vegetative growth. It is further suggested that complex hydrographic conditions in the East China Sea result in a dynamic bloom with contributions by both autochthonous and allochthonous species.Communicated by T. Ikeda, Hakodate     

Geostatistical analysis of spatial distribution of Liocarcinus depurator,Macropipus tuberculatus and Polybius henslowii (Crustacea: Brachyura) over the Galician continental shelf (NW Spain)

Geostatistics was used to analyze the spatial structure and distribution of three species of brachyurans, Liocarcinus depurator, Macropipus tuberculatus and Polybius henslowii, from sets of data collected during three survey cruises (1983 and 1984) over the Galician continental shelf. The present study investigates the feasibility of using geostatistics with data collected according to traditional methods and of thereby improving the sampling methodology. In order to investigate the spatial structure of the species studied, experimental variograms were calculated and fitted to a spherical model. The spatial structure model was used to estimate and map abundance and distribution of the populations studied using the so-called Kriging technique. Geostatistical analysis enabled the determination of spatial density gradients as well as patch size (14 to 22 km, L. depurator; 10 to 28 km, M. tuberculatus; 7.5 to 28 km, P. henslowii) along the continental shelf. Depth was revealed as a limiting factor, restricting the distribution of L. depurator and M. tuberculatus on a large scale, whereas upwelling processes and nutrient-rich waters from the rías affected the spatial structure on a smaller scale, especially in the case of P. henslowii. A spatial segregation in the distribution of the three species also emerged; this probably arose from differences in physical and biological factors that result in different habitat-exploitation patterns. The study demonstrates the existence of spatial covariance and that the variograms vary as a function of population density and geographical area. This information will be useful in improving the design of future sampling cruises.     

Distribution of western beaufort sea polychaetous annelids

Depth and depth-related processes appear to exert primary control over the distributional patterns of species of polychaetous annelids in the western Beaufort Sea (Cape Halkett to Barter Island, Alaska). Species richness and total polychaete abundance are maximal along the outer continental shelf and upper continental slope. Stations exhibiting the most similar polychaete assemblages are located at similar depths. However, some species distributions appear to correlate better with sediment type than with depth. Maximum abundance occurs deeper on the continental slope to the west, and station clusters generated by non-hierarchical clustering (using the dominant polychaete species data) are not sorted strictly by depth. In a canonical analysis of discriminance, the station clusters were projected onto a two-dimensional plane in species space. The first and second canonical variables of the station clusters correlate with sediment grain-size distributions, suggesting a relationship between polychaete distribution patterns and the sedimentary environment. This relationship is further substantiated when sediment grain-size distributions for each station are plotted on a tertiary diagram: the stations are grouped and ordered in a pattern similar to that generated by the canonical analysis of discriminance.     

Gulf of Mexico hydrocarbon seep communities

Sediment and water samples were collected by submersible in September 1986 at 16 locations on the carbonate cap overlying a conical diapir, which was formed by the upward migration of oil and gas through a subsurface fault on the continental slope off Louisiana, USA (27°47N; 91°30.4W). The biological community at the site was photographed quantitatively with still and video cameras. Rigorous spatial sampling indices were maintained so that variation in chemical parameters and in the abundance of photographed organisms could be estimated within the bounds of the study site. Concentrations of extractable organic material (EOM) ranged from 0.24 to 119.26 in the sediment samples, while methane concentrations in the water samples were from 0.037 to 66.474 M. The visible biological community was predominantly composed of the chemosynthetic tube worms (Vestimentifera) Lamellibrachia sp. and Escarpia sp., and an undescribed, methane-oxidizing mussel (Mytilidae: Bathymodiolus-like), as well as diverse non-chemosynthetic organisms. The ranked abundance of tube worms was significantly correlated (p<0.05) with the concentration of EOM in the sediment samples, while the abundance of mussels was significantly correlated (p<0.05) with the concentration of methane in the water samples. Tube worms and mussels both occurred in dense clusters; however, the clusters of mussels had a more restricted distribution within the study site than did clusters of tube worms. Both organisms were most abundant in the vicinity of the subsurface fault.     

Food habits and intestinal parasites of deep demersal fishes from the upper continental slope east of Newfoundland,northwest Atlantic Ocean

Stomach contents and intestinal parasite faunas of 471 individuals of demersal fishes in 14 species were examined from the Carson Canyon region (Lat. 45°30N; Long. 48°40W) of the upper continental slope of the Grand Banks off Newfoundland, Canada. Individual species tended to feed either on benthic or on pelagic/benthopelagic organisms, but pelagic prey assumed the greatest importance overall. Data from stomach contents were supported by the parasite information. Prevalence of parasites was higher in benthic feeders (53.1%) than in pelagic feeders (28.9%), and relative abundance by major group was: Digenetic Trematoda 5.8% benthic vs 27.8% pelagic, Nematoda 53.1% vs 72.2%, and Acanthocephala 40.9% vs 0%. Of the dominant fishes, there were more species of benthic feeders (5) than pelagic feeders (3), but pelagic feeders were numerically more abundant (pelagic 70.9%, benthic 20.5%). Benthic feeders were on average larger (=270.6g) than pelagic ones (=130.6g), but pelagic feeders represented a larger proportion of the biomass (pelagic 43.3%, benthic 25.9%). The results of this study combined with those from other areas suggest that feeding from the pelagial by demersal fishes at upper continental slope depths is probably the general rule.     

Post-larval development of the common north-west European brittle stars Ophiura ophiura,O. albida and Acrocnida brachiata (Echinodermata: Ophiuroidea)

The post-larval skeletal plate ontogeny of the ophiuroids Ophiura ophiura (Linnaeus) and Acrocnida brachiata (Montagu) from the recently metamorphosed 0.3 mm disc diameter size to a juvenile size of 2.0 mm disc diameter has been examined using scanning electron microscopy. Post-larval development of O. albida Forbes is briefly described for sizes between 1.0 and 2.0 mm disc diameter. Early post-larval development in the 0.2 to 1.0 mm discsize range involves considerable morphogenetic change and necessitates the use of a developmental series in the identification of newly settled ophiuroids. Comparison of these ophiuroids with other closely related species reveals that the post-larvae are readily distinguishable at even the smallest sizes by their transient post-larval morphologies. Abbreviated larval development in A. brachiata is suggested from the egg size and post-larval form. The ophiuroid post-larvae were collected from the shallow sublittoral of Oxwich Bay, Bristol Channel, UK, during the years 1980 to 1982.     

Diets of some deep-sea brittle stars in the Rockall Trough

The stomachs of 1 165 individuals of the species Ophiacantha bidentata (Retzius), Ophiactis abyssicola (M. Sars), Ophiocten gracilis (O. Sars), Ophiura irrorata (Lyman), O. ljungmani (Lyman) and Ophiomusium lymani Wyville Thomson, collected from the general area of the Rockall Trough from 1973 to 1983, were examined; 47% contained material. This varied in nature and in volume between individuals, but overall, the diets of the 6 species showed quite high similarity. The few differences evident are interpreted as reflecting different lifestyles. Except for Ophiocten gracilis, the probably motile, epifaunal species (Ophiura irrorata, O. ljungmani and Ophiomusium lymani) showed the greatest variety in items, and the lowest similarity with the remaining species; they are probably unselective omnivores, eating whatever small prey or organic detritus they are able to find. Ophiacantha bidentata and Ophiactis abyssicola showed both the least dietary variety, and the highest similarity to each other. This probably reflects their more sedentary lifestyle, feeding on current-borne particles and small prey from perches on sessile fauna. The stomach contents of Ophiocten gracilis consisted mainly of amorphous organic material and mineral particles, indicating a microphagous feeding mechanism. With O. gracilis, the proportion of stomachs with food decreased markedly in summer, this coinciding with seasonal gonadal development when stomachs frequently appeared ruptured or compressed. The possible importance of seasonal sedimentation of fast-sinking particulates from the surface is discussed in relation to finds of flocculent organic material, often containing diatom frustules, in stomachs of all 6 species. Some doubts remain as to whether our results are biased by specimens egesting stomach contents on capture. As in shallow water, deep-sea ophiuroids seem to be trophic generalists lacking in dietary specialization. Brief notes on parasites encountered are also given.     

Growth,production and ecological significance of Ophiura albida and O. ophiura (Echinodermata: Ophiuroidea) in the German Bight

Growth and production of the shallow-water ophiuroids Ophiura albida and O. ophiura were investigated at two stations in the German Bight from 1988 to 1991. Growth rings visible on the vertebral ossicles of the ophiuroid arms were interpreted as annual age markers. A correction for overgrown first rings allows for more exact estimations of growth and age. In both species growth could be described by Von Bertalanffy growth functions with the asymptotic disc diameter D =10.1 mm, K=0.229 and t _o=-0.192 in O. albida and D =27.7 mm, K=0.084 and t _o=0.042 in O. ophiura. Somatic production was calculated from mass specific growth rates. Annual production:biomass (P:B) ratios were estimated at 0.32 for O. albida and 0.43 for O. ophiura.AWI Publication Number: 618     

Skeletal growth markers in the deep-sea brittle starsOphiura ljungmani andOphiomusium lymani

Growth bands have been found in the calcitic vertebral arm ossicles of the commonly occurring deep-sea brittle starsOphiura ljungmani Wyville Thomson andOphiomusium lymani (Lyman) (Echinodermata: Ophiuroidea) trawled from the Rockall Trough (N.E. Atlantic) at 2 200 and 2 900 m depth from 1973 to 1982. InOphiura ljungmani, the study of ossicle microstructure by SEM shows that growth bands reflect differences in stereom porosity and surface relief, similar to that previously found amongst shallow-water brittle stars. The pattern inOphiomusium lymani was much less clear from the microstructure, but could be revealed by heating the ossicle to 450 °C and clearing in xylene. The bands showed up as fine, translucent rings separated by more opaque, wider zones, perhaps reflecting differences in organic material incorporated within the calcite. Both growth-banding patterns probably reflect an annual cycle in skeletal growth rate. On this assumption, the number and spacing of the banding in the two species indicates contrasting growth strategies. WithOphiura ljungmani, a rather regular annual growth increment and perhaps shorter lifespan (up to ca. 10 yr) thanOphiomusium lymani is indicated. The latter shows a relatively wide spacing of early bands, followed by tight clustering of the outermost bands corresponding to adult sizes. This growth pattern is characteristic of species escaping from predation by rapid growth to relatively large adult size. AdultO. lymani probably grow slowly, some perhaps reaching 20 yr of age. Growth curves were fitted to size-at-age corresponding to measurements of the size and ordering of growth bands. These corroborate age structure previously estimated from analysis of size frequencies in time series from the stations sampled in the present study. Such skeletal growth markers should be of value in analysis of the demography of deep-sea populations.     

Biodiversity in the ecosystem of the Portuguese continental shelf: distributional ecology and the role of benthic amphipods

The biodiversity and distributional ecology of amphipod crustaceans were analysed from benthic surveys in 1985 and 1986 along the coast of Portugal, from the Tagus canyon to the coast of Minho, in order to determine their role in the ecosystem of the Portuguese continental self. In samples obtained from different types of substrate between 18 and 545 m, we identified 113 species belonging to 51 genera of 24 families. Seventeen of these species were new records for the Portuguese coast. The relative frequencies and number of individuals of all species were analysed. The genus Ampelisca clearly dominated the group of the most frequent and abundant species, which also exhibited a large range of vertical distribution. Most of the 113 species were found only down to 150 m depth, corresponding roughly to the lower limit of the continental shelf. Fifteen species exhibited clear bathyal traits, being consistently found deeper than 100 m, especially in the Tagus canyon and the upper zone of the continental slope. The most frequent and abundant species were found predominatly or in large numbers in medium to fine sand bottoms. This type of substrate also contained the highest number of species. The granulometric structure of the habitat could constitute the most important factor controlling both biodiversity and development of large amphipod populations, since oxygen level, water exchange, number of available shelters, and probably the quantity and quality of nutritional resources all vary as a function of substrate texture. Because of their dominance in both frequency and abundance, several Ampelisca species may constitute an important food source for many secondary consumers of the shelf sediments, especially for demersal fishes.     

Distribution of the cladoceran Podon polyphemoides in the Chesapeake Bay

The distribution of the cladoceran Podon polyphemoides (Leuckart) in the Chesapeake Bay (USA) estuarine system was determined by a quantitative pump sampling method, and the patterns of abundance were correlated with temperature and salinity distributions. The species was seasonally recurrent, with distinct population maxima in the central portion of the bay. Population densities in excess of 60,000 podonids/m³ have been recorded. The podonids first appeared in the spring in the shallow tributaries, when water temperatures near the bottom reached 6°C. The vernal populations disappeared when summer temperatures exceeded 27°C, but reappeared in the fall as the water cooled. The species was euryhaline and eurythermal in its distribution, but the greatest concentrations were attained within relatively narrow zones of temperatures between 11^o and 26°C, and salinities between 8 and 18. The production of males, sexual females and sexual eggs occurred both in the spring and the fall between the thermal limits of 11^o and 17°C.     

Spatial patterns of ichthyoplankton distribution in relation to hydrographic features in the Northern Benguela region

The spatial distribution of the most abundant eggs and larvae of teleost fish species on the continental shelf and slope off the northern Benguela region was studied in April 1986. The horizontal and vertical distribution of eggs and larvae were analysed together with environmental data, in order to determine patterns of ichthyoplankton distribution. Both species composition and relative egg and larval abundance levels exhibited important latitudinal differences during a period of quiescent upwelling with an intense intrusion of Angolan water into the system. Larval diversity was higher in the northern part of the study area, where, because of the intrusion of the warmer Angolan water, the water column was more stratified than in the southern part, where the affect of upwelling of South Atlantic Central Water was continuous and only a few species spawned. The frontal zone appeared to be a nursery ground for the most important pelagic species of the region:Trachurus trachurus capensis, Engraulis capensis, andSardinops ocellatus. Vertical egg and larval distributions showed evidence of stratification, with highest concentrations located in the uppermost 50 m. In comparison, during periods of intense upwelling, longitudinal gradients were responsible for the horizontal distribution of ichthyoplankton, and the vertical distribution of eggs and larvae were much more extensive because of the greater mixing of the water column.     

Cross-shelf distribution of copepods and fish larvae across the central Great Barrier Reef

The distribution of total dry weight of zooplankton, copepod numbers and ichthyoplankton across the outer continental shelf in the central Great Barrier Reef was examined at bi-weekly intervals for three months over summer of 1983. Copepods were sampled (236 m net) within 10 m of the surface and within 10 m of the bottom. Mean densities in surface waters decreased markedly from the mid-shelf to outer shelf and the Coral Sea, but no cross-shelf gradient occurred in the bottom-water. Densities of copepods on the mid-shelf (surface and bottom waters) and in bottom-waters of the outer shelf were typically ca. 400 m^–3. Significantly lower densities (ca. 100 m^–3) occurred in surface waters of the outer shelf, except during outbursts of Acartia australis, when densities in these waters differed little from those elsewhere on the shelf. In oceanic waters, 10 km from the outer shelf station, copepod densities in surface waters were ca. 40 m^–3. Four of the five most abundant copepod taxa in surface waters, Paracalanus spp., Eucalanus crassus, Acrocalanus gracilis and Canthocalanus pauper, tended to be most abundant at the mid-shelf end of the transect. Acartia australis was sporadically very abundant in surface waters of the outer shelf, as was Paracalanus spp. in bottom-water of the outer shelf. An assemblage of Coral Sea species of copepod occurred in bottom-water of the outer shelf during two major intrusions, but not at other times. Densities of all common species varied considerably between cruises. Maximum densities of all common species except A. australis tended to be associated with diatom blooms linked to intrusions but a bloom did not necessarily mean all common species were abundant. Fish larvae included both reef and non-reef taxa, with reef taxa predominating on the outer shelf (approx 2:1 in density of individuals) and non-reef taxa dominating in nearshore samples (approx 2:1). Nine of the ten most abundant taxa analysed showed highly significant variation in numbers among stations and all but one of these also exhibited significant station x cruise interactions. Interactions generally reflected changes in the rank importance of adjacent stations from one cruise to the next or lack of any significant cross-shelf variation on some cruises where overall abundance of the taxa was low.     

Distribution and abundance of fish larvae off the coast of West Pakistan

The material was collected during 4 different cruises on the shelf off the coast of W. Pakistan in November–December, 1964 and in March, 1967 and 1968. The hydrography as well as the zooplankton-biomass distribution are briefly described. A total of 5777 larval fish were taken in 33 positive vertical hauls by an Indian Ocean Standard Net. The number of larvae in positive hauls ranged from 2 to 1262 larvae haul^-1. The most productive area of the ichthyoplankton was the waters south of Karachi, covered during the Machera Cruise, where 828 to 1262 larvae haul^-1 were obtained. The larvae were identified to species, genera or family. Sardinella sindensis (specific identification uncertain) larvae dominated in abundance. The other abundant larvae belonged to Benthosema spp., Amentum commersonii, Vinciguerria spp. and Diaphus spp. Larval distributions and abundances are described. Aggregation and spawning of adult Sardinella (sindensis) in the waters off the coast of W. Pakistan in November/December, 1964, are discussed. Presumably, the waters south of Karachi represent good feeding grounds for s. (sindensis) larvae. Displacement volume, length and weight relationships of various larval size groups have been studied and a high rate of larval mortality (at a length of 5.0 to 8.5 mm) observed.