Activity and metabolism of larval Atlantic cod (Gadus morhua) from Scotian Shelf and Newfoundland source populations

Patterns of activity and metabolism were investigated in larval Atlantic cod (Gadus morhua L.) between December 1991 and July 1992: (1) throughout larval development; (2) between two genetically discrete populations (Scotian Shelf and Newfoundland) and (3) as a function of two different culture temperatures. During the yolk-sac stage (0 to 5 d post-hatch), changes in swimming speed were not related to mass-specific metabolic rates; no portion of the mass-specific oxygen consumption could be explained by changes in activity. In the mixed feeding stage (6 to 14 d posthatch), there was a tendency for oxygen consumption to be related to changes in swimming speed. In the exogenous feeding stage (>14 d post-hatch), oxygen consumption significantly increased with swimming speed. These ontogenetic patterns of activity and metabolism were the same for larvae from the Scotian Shelf and Newfoundland populations. However, over the entire larval life and among ontogenetic stages, the metabolic cost of activity (mass-specific O₂ consumption/swimming speed) of Scotian Shelf larvae was significantly higher than that of Newfoundland larvae. When cod larvae, that had developed at 5°C, were acutely exposed to 10°C, Scotian Shelf larvae had a higher intrinsic cost of activity than Newfoundland larvae, over the entire larval life. During the exogenous feeding stage, the mean metabolic cost of activity for Newfoundland larvae raised at 10°C and tested at 10°C was significantly higher and more variable than that of larvae raised at lower temperatures. However, the metabolic cost of activity of larvae raised and tested at 10°C was not significantly different between source populations. Together these findings suggest that differences in swimming energetics reflect changing energy requirements for activity among ontogenetic stages, and reflect adaptation to regional environments among genetically discrete populations.     

Egg buoyancy variability in local populations of Atlantic cod (Gadus morhua)

Previous studies have found strong evidences for Atlantic cod (Gadus morhua) egg retention in fjords, which are caused by the combination of vertical salinity structure, estuarine circulation, and egg specific gravity, supporting small-scaled geographical differentiations of local populations. Here, we assess the variability in egg specific gravity for selected local populations of this species, that is, two fjord-spawning populations and one coastal-spawning population from Northern Norway (66–71°N/10–25°E). Eggs were naturally spawned by raised broodstocks (March to April 2009), and egg specific gravity was measured by a density-gradient column. The phenotype of egg specific gravity was similar among the three local populations. However, the associated variability was greater at the individual level than at the population level. The noted gradual decrease in specific gravity from gastrulation to hatching with an increase just before hatching could be a generic pattern in pelagic marine fish eggs. This study provides needed input to adequately understand and model fish egg dispersal.     

Condition of larval cod (Gadus morhua) off southwest Nova Scotia in 1983 in relation to plankton abundance and temperature

In our study of the condition of larval cod (Gadus morhua) collected off southwest Nova Scotia in winter-spring, 1983, we (1) examined relationships between larval condition and ambient environmental conditions, and (2) compared the use of simple morphometric indices of larval condition and of multivariate statistics to obtain information relevant to larval condition. Twelve indices of relative condition were obtained, based upon seven measurements made on each larva, and a principal component (PC) analysis was performed on these condition indices. Most condition indices and the first PC were significantly correlated with numbers of nauplii and of zooplankters per m³ in the water column, which are direct measures of food available to the larvae, but not to less direct measures of environmental quality for the larvae, such as plankton displacement volume or chlorophyll concentration. Conventional indices based upon relations of dry weight or body height at the anus to length were most sensitive to environmental conditions. There was no apparent advantage in performing a multivariate analysis based upon a larger series of measurements.     

Foraging behaviour of larval cod (Gadus morhua) influenced by prey density and hunger

Fish larvae meet diverse environmental conditions at sea, and larval growth and chance of survival depend on a flexible response to environmental variability. The present study focuses on the flexibility of the foraging behaviour of larval cod in a series of laboratory experiments on larval search activity, prey selectivity, and hunger in a variable prey environment. Gadus morhua eggs were collected in March 1992 and 1993 from the Kattegat area, Denmark, fertilised and incubated in the laboratory. After hatching, the larvae were transferred to rearing tanks of 172 litres. The behaviour of larvae (6 to 7 mm long) was observed visually, and prey attacks, swimming activity and gut contents were registered across a range of 1 to 120 copepod nauplii l^-1. When prey density decreased, larvae increased their swimming activity, increased their responsiveness to prey (distance of reaction) and decreased their prey size selectivity. Behavioural response was to a large degree determined by the level of hunger, represented by the number of newly ingested prey in the gut. The findings show that cod larvae have a flexible response to changes in feeding conditions and imply that larvae can grow and survive even in the lower range of (mean) prey densities measured at sea.     

Sub-lethal exposure to ultraviolet radiation reduces prey consumption by Atlantic cod larvae (Gadus morhua)

Levels of ultraviolet-B radiation (UV-B: 280–315 nm) reaching the earth’s surface have increased over the past few decades due to ozone depletion. It is well documented that exposure to UV-B radiation increases mortality in marine fish larvae. However, few studies have examined sub-lethal effects of UV-B radiation such as, for example, the possibility that it affects prey consumption by fish larvae. Atlantic cod larvae were exposed to a sub-lethal level of ultraviolet-B radiation (UV-B: 280–315 nm) for 15 h. After the exposure, rotifers (4/mL) were fed to cod larvae at three different post-exposure intervals (20 min, 3 and 6 h). Trials were replicated three times for each post-exposure interval. The number of rotifers in the gut and the percent of empty guts (number of fish with empty gut/number of fish examined × 100) were analyzed. Results were compared to those of unexposed fish (the control treatment). UV-B exposed cod larvae had consumed significantly fewer rotifers than control fish at all post-exposure intervals. There was no significant difference in the frequency of empty guts between fish in the UV-B treatment versus fish in the control treatment at any of the post-exposure time points (although the difference at 20 min post-exposure was borderline significant). These observations suggest that cod larvae exposed to sub-lethal levels of UV-B have lower net energy gain which may lead to reduced growth rate and possibly poorer survival.     

The swimming kinematics of larval Atlantic cod, Gadus morhua L., are resilient to elevated seawater pCO2

Kinematics of swimming behavior of larval Atlantic cod, aged 12 and 27 days post-hatch (dph) and cultured under three pCO₂ conditions (control-370, medium-1800, and high-4200 μatm) from March to May 2010, were extracted from swim path recordings obtained using silhouette video photography. The swim paths were analyzed for swim duration, distance and speed, stop duration, and horizontal and vertical turn angles to determine whether elevated seawater pCO₂—at beyond near-future ocean acidification levels—affects the swimming kinematics of Atlantic cod larvae. There were no significant differences in most of the variables tested: the swimming kinematics of Atlantic cod larvae at 12 and 27 dph were highly resilient to extremely elevated pCO₂ levels. Nonetheless, cod larvae cultured at the highest pCO₂ concentration displayed vertical turn angles that were more restricted (median turn angle, 15°) than larvae in the control (19°) and medium (19°) treatments at 12 dph (but not at 27 dph). Significant reduction in the stop duration of cod larvae from the high treatment (median stop duration, 0.28 s) was also observed compared to the larvae from the control group (0.32 s) at 27 dph (but not at 12 dph). The functional and ecological significance of these subtle differences are unclear and, therefore, require further investigation in order to determine whether they are ecologically relevant or spurious.     

Movements of farmed and wild Atlantic cod (Gadus morhua) released in Trinity Bay,Newfoundland

Fishery scientists and managers are investigating the feasibility of enhancing annual recruitment to the northern Atlantic cod (Gadus morhua L.) stock complex off Labrador and northeastern Newfoundland through the release of farmed fish back into the sea. Release of newly matured fish and adults with farm-advanced fecundity would increase the spawning biomass. Enhancement efforts might be measurably successful in major bays that are year-round habitats for cod. To determine if farmed cod would remain and spawn in Trinity Bay, 14 fish with surgically implanted transmitters were released in November 1992. Sonic tracking confirmed that farmed cod released on the western side of Trinity Bay overwintered within the bay, and integrated with wild cod approaching spawning condition in April 1993. Blood plasma antifreeze levels confirmed that these wild cod had overwintered inshore in subzero waters. A spawning aggregation was found in July 1993, providing evidence that northern cod reproduce in Newfoundland bays. These findings suggest that it may be possible to increase the number of cod spawning inshore through the release of farmed fish.     

How does cod (Gadus morhua) cope with variability in feeding conditions during early larval stages?

Response of mesocosm-reared cod (Gadus morhua L.) larvae to different feeding conditions was investigated in 1988 in two mesocosms: a large basin and a smaller bag enclosure within the basin. The basin was filled with seawater, and a community of naturally occurring plankton developed. Plankton concentrations were monitored, and cod larvae stocked in the enclosures were sampled for determination of growth, survival, and gut content. In the bag, insufficient amounts of energetically favourable prey, as copepod nauplii, led to non-selective ingestion of plankton from a broad range of sizes, including considerable amounts of protozoans (tintinnid and oligotrich ciliates). Growth of larvae from the bag was low, with daily specific growth rates (SGR) less than 2.8% the first 3 wk post-hatch. This was followd by rapid increase of SGR to 21.7%, which coincided with a large increase in availability of copepod nauplii. In the basin, high nauplii concentrations led to SGR of 13.7 to 21.7% from onset of feeding to 16 d post-hatch, respectively. Under such conditions, the larvae were highly selective feeders. At 3 wk post-hatch, survival was 36.7 and 38.3% in the basin and bag enclosure, respectively. To cope with variations in the feeding conditions, the cod larvae were shown to be opportunists when nauplii were scarce, and included plankton from several trophic levels in their diet. When nauplii were abundant, cod larvae realized their high potential for growth. Both opportunism and realization of a high growth potential may enhance survival of the larvae.     

Residency and depth movements of a coastal group of Atlantic cod (Gadus morhua L.)

Atlantic cod stocks are subdivided into geographically and biologically discrete populations; some being locally resident, others migratory. The reasons for the variation in migration pattern and the consequences it may have for population structuring is poorly understood. We studied a group of cod from the coastal waters of the Shetland Isles in the northern North Sea. During the spawning season, electronic tags that record depth and temperature over time were implanted into 133 individuals and the fish released within a few kilometres of where they were captured. Thirty-nine cod have been recaptured up to 608 days later, throughout the year and, in all but two cases, within 15 km of their release site. Geolocation methods based on temperature and bathymetry also suggested that the cod had a limited home range, remaining resident year-round in coastal waters. The cod were deeper during the winter and moved to shallower water in late spring and summer. They showed diel, fortnightly and monthly cycles in depth movement that varied much within and between individuals over the season. Residency and the individual variability in vertical movement may reflect a combination of locally complex depth strata, variable seabed substrate and the wide range of seasonally available prey resources.     

Habitat use by juvenile Atlantic cod (Gadus morhua) in the presence of an actively foraging and non-foraging predator

Experiments were conducted in the autumn and winter of 1992/93 to examine habitat use by juvenile (age 0+) Atlantic cod, Gadus morhua L., before, during and following exposure to a passive or actively foraging predator (age 3+ cod). Experiments presented groups of juvenile cod (n=5 fish/group) with one of two combinations of three substrates; (1) gravel, sand, and a patch of artificial kelp (kelp), or (2) cobble, sand, and kelp. Cobble is known to provide juvenile cod with a refuge from predation. Kelp was used to test the hypothesis that juvenile cod associate with fleshy macroalgae in nature because of the safety it provides from predators. There was little difference in habitat use by juvenile cod before, during or following exposure to a passive predator. Under these conditions, juvenile cod appeared to prefer finer grained mineral substrates and avoided the kelp. The extent of the juvenile response to a passive predator was to avoid the predator's location in the experimental tank. In contrast, juvenile cod showed a significant shift in habitat use when exposed to an actively foraging predator, hiding in cobble or, when cobble was not available, in kelp. Use of both these habitats resulted in a significant reduction in predation risk to the juvenile cod. Our results suggest that: (1) an association with kelp provides safety from predation to juvenile cod, and (2) juvenile cod are capable for assessing the risk a predator represents and adjust their response accordingly.     

Feeding opportunities of larval and juvenile cod (Gadus morhua) in a Greenlandic fjord: temporal and spatial linkages between cod and their preferred prey

Feeding of fish depends on a spatial and temporal match with prey, and since larval and juvenile feeding can be highly selective, their preferences for given prey sizes and taxa should be considered when quantifying the actual availability of potential prey. We investigated the diet and prey preferences of the early-life stages of Atlantic cod (Gadus morhua) to quantify the availability of prey during a spring-summer season in a West Greenlandic fjord. We hypothesized that abundances of larval and juvenile cod at size were synchronized to optimal availability of preferred prey in space and time. The present analysis is based on nine cruises each covering 5 stations visited between 24 May and 5 August 2010 comparing zooplankton abundance, cod gut content and distribution patterns. Cod 4–25 mm in length preferred prey of about 5 % of their own length. During ontogeny, their preferences changed from calanoid nauplii towards Pseudocalanus spp. and Calanus spp. copepodites. The larvae/juvenile had an exceptionally high dietary contribution from cladocerans, which were highly preferred by cod larger than 9 mm, while the abundant Metridia longa and the non-calanoid copepods contributed less. These findings stress the importance of focusing on abundance of preferred prey when assessing the actual prey availability to young fish. We found a spatio-temporal overlap between cod and their preferred prey, and observations suggest that advection of both zooplankton and cod contributed to this overlap. Hence, the larval feeding opportunities might be sensitive to climate-related changes affecting the circulation patterns in this fjord.     

Maternal allocation of lipid classes and fatty acids with seasonal egg production in Atlantic cod (Gadus morhua) of wild origin

A suite of characteristics is often used to assess egg quality as these properties potentially play important roles in progeny survival and growth. Our objective was to assess egg characteristics including lipid biocomposition of an iteroparous, batch-spawning teleost of wild origin. Maternal allocation to egg number was generally dome-shaped (5 of 8 females) and egg size declined over the breeding season for eight breeding pairs of wild Atlantic cod (Gadus morhua) (n = 43 batches). Egg lipid composition ranged considerably among females and between egg batches within females (e.g., phospholipids 40–86 %; polar 47–87 % and neutral lipids 15–52 % of total lipids; polyunsaturated fatty acids 16–50 % of total fatty acids). Principal component analyses revealed significant inter-relationships among maternal traits, batch sequence and fecundity, and egg size and composition. Seasonal trends with regard to lipid deposition were variable; three females showed consistent declines in lipid parameters (μg egg⁻¹) with both batch number and egg diameter, one female showed consistent increase and the four remaining females showed no trend. The three females that exhibited seasonal declines in egg lipid content were characterized as having high fertilization success (>75 %). Our findings highlight the variability in lipid allocation to eggs of batch spawners of wild origin and characterize the composition of endogenous reserves available during embryogenesis and yolk sac larval stages.     

Egg mortality and hatching rate of Baltic cod (Gadus morhua) in different salinities

Egg mortality of Baltic cod (Gadus morhua L.), collected off northern Gotland, Sweden, in 1990, was studied in four different salinities — 10 and 15 ppt (salinity of the principal spawning areas of Baltic cod) and 5 and 7 ppt (salinity above the halocline) — in laboratory experiments. Mortality was high during the first 4 d of development, but after gastrulation mortality was low in all salinities tested, except for 5 ppt, in which mortality increased slightly before hatching. Mortality during hatching varied considerably with salinity. No hatching occurred in 5 ppt salinity, and only a few larvae survived in 7 ppt salinity; in contrast, mortality during hatching was comparatively low in salinities of 10 and 15 ppt.     

Egg buoyancy of Baltic cod (Gadus morhua) and its implications for cod stock fluctuations in the Baltic

Successful spawning of Baltic cod (Gadus morhua L.) is restricted to the deep basins of the Baltic Sea and is dependent on abiotic conditions such as salinity and water oxygen content. Due to irregular inflows of saline water from the North Sea, the conditions for spawning fluctuate and consequently so does the cod stock. In May and June 1990 eggs were obtained from spawning cod caught off northern Gotland, Sweden. Our investigation revealed neutral egg buoyancy for Baltic cod at a salinity of 14.4±1.1 ppt, with a slight decrease some days before hatching. Today salinities of this magnitude occur only in the Bornholm basin, whereas in the Gdansk and Gotland basins the eggs sink toward the bottom and are exposed to lethal oxygen conditions.     

Foraging behavior in early Atlantic cod larvae (Gadus morhua) feeding on a protozoan (Balanionsp.) and a copepod nauplius (Pseudodiaptomussp.)

Food limitation is likely to be a source of mortality for fish larvae in the first few weeks after hatching. In the laboratory, we analyzed all aspects of foraging in cod larvae (Gadus morhua Linnaeus) from 5 to 20 d post-hatching using protozoa (Balanion sp.) and copepod nauplii (Pseudodiaptomus sp.) as prey. A camera acquisition system with two orthogonal cameras and a digital image analysis program was used to observe patterns of foraging. Digitization provided three-dimensional speeds, distances, and angles for each foraging event, and determined prey and fish larval head and tail positions. Larval cod swimming speeds, perception distances, angles, and volumes increased with larval fish size. Larval cod swam in a series of short intense bursts interspersed with slower gliding sequences. In 94% of all foraging events prey items were perceived during glides. Larval cod foraging has three possible outcomes: unsuccessful attacks, aborted attacks, and successful attacks. The percentage of successful attacks increased with fish size. In all larval fish size classes, successful attacks had smaller attack distances and faster attack speeds than unsuccessful attacks. Among prey items slowly swimming protozoans were the preferred food of first-feeding cod larvae; larger larvae had higher swimming speeds and captured larger, faster copepod nauplii. Protozoans may be an important prey item for first-feeding larvae providing essential resources for growth to a size at which copepod nauplii are captured. Received: 20 April 1999 / Accepted: 12 January 2000     

RNA/DNA ratio and expression of 18S ribosomal RNA, actin and myosin heavy chain messenger RNAs in starved and fed larval Atlantic cod (Gadus morhua)

Levels of total RNA, total DNA, 18S ribosomal RNA (rRNA), poly(A) messenger RNA (mRNA), and two mRNAs coding for abundant myofibrillar proteins were estimated in laboratory-reared Atlantic cod larvae (Gadusmorhua Linnaeus) under conditions of feeding and starvation. DNA probes specific for cod 18S rRNA, β-actin mRNA and myosin heavy chain mRNA were developed. In two experiments on newly hatched larvae in fed and starved treatments, changes in 18S rRNA and mRNA were similar to changes in total RNA during the first weeks after hatching. RNA levels in fed and starved larvae in both experiments were stable, or increased, over the first 3 d after hatching, and then decreased to minima at 9 d. RNA levels increased after 9 d, with the degree and timing of the increase varying among the individual classes of RNA. Complete mortality of starved larvae in both experiments was observed shortly after 11 d, corresponding to exhaustion of endogenous yolk reserves. Total RNA content, RNA/DNA ratio, 18S rRNA levels, total mRNA pool, and actin and myosin heavy chain mRNA levels showed significant differences in fed and starved first-feeding larvae after yolk exhaustion. In another experiment with 3- to 4-week-old cod larvae, 18S rRNA levels were significantly lower in starved versus fed larvae after 3 d. Total RNA responded to feeding and starvation within a similar time as 18S rRNA and the mRNAs examined. Analysis of bulk nucleic acids using fluorometric dyes was simpler and faster than analysis of individual RNAs using hybridization probes, and provides valuable information on recent growth and condition of individual larvae. However, analysis of specific RNAs can provide information on expression of the corresponding genes and reveal the changes underlying trends seen in bulk RNA. Received: 9 February 1996 / Accepted: 7 June 1999     

Variation in condition of coastal Newfoundland 0-group Atlantic cod (Gadus morhua): field and laboratory studies using simple condition indices

Demersal 0-group Atlantic cod (Gadus morhua L.) were collected in Trinity Bay, Newfoundland, Canada, between August 1993 and January 1994. Diet composition and simple indices of condition (hepato-somatic index, condition factors and body ash content) were determined. Laboratory experiments were also conducted to determine whether simple indices of condition reflect differences in energy intake (ration level) and to determine minimum index of condition values, assessed from 0-group cod that died due to exhaustion of their energy reserves. Determination of lower critical indices of condition provided a meaningful interpretation of field data on feeding, condition and survival of 0-group cod. In the wild, indices of condition reflecting variation in liver and muscle energy reserves of cod increased rapidly with the consumption of Calanus finmarchicus, which exhibited a high incidence of oil sacs, and declined abruptly when this lipid-rich prey was no longer consumed. Liver and muscle condition of 0-group cod from the field varied seasonally, but their condition was consistently above that of cod that died in the laboratory. Laboratory studies, using three ration levels, revealed that condition indices differed over time and among ration levels. Indices of condition representing variation in muscle energy reserves exhibited a steady decline once the hepato-somatic index fell below about 2.0. We interpret this as evidence for depletion of lipids first and then proteins, a pattern similar to that of larger juveniles and adults. We conclude that as long as 0-group cod are above a “safe” level of condition their body length can still increase even when condition is declining. Received: 2 September 1997 / Accepted: 8 December 1998     

Egg and early larval stages of Baltic cod, Gadus morhua, are robust to high levels of ocean acidification

The accumulation of carbon dioxide in the atmosphere will lower the pH in ocean waters, a process termed ocean acidification (OA). Despite its potentially detrimental effects on calcifying organisms, experimental studies on the possible impacts on fish remain scarce. While adults will most likely remain relatively unaffected by changes in seawater pH, early life-history stages are potentially more sensitive, due to the lack of gills with specialized ion-regulatory mechanisms. We tested the effects of OA on growth and development of embryos and larvae of eastern Baltic cod, the commercially most important fish stock in the Baltic Sea. Cod were reared from newly fertilized eggs to early non-feeding larvae in 5 different experiments looking at a range of response variables to OA, as well as the combined effect of CO₂ and temperature. No effect on hatching, survival, development, and otolith size was found at any stage in the development of Baltic cod. Field data show that in the Bornholm Basin, the main spawning site of eastern Baltic cod, in situ levels of pCO₂ are already at levels of 1,100 μatm with a pH of 7.2, mainly due to high eutrophication supporting microbial activity and permanent stratification with little water exchange. Our data show that the eggs and early larval stages of Baltic cod seem to be robust to even high levels of OA (3,200 μatm), indicating an adaptational response to CO₂.     

Effect of temperature on viability and axial muscle development in embryos and yolk sac larvae of the Northeast Arctic cod (Gadus morhua)

Cod (Gadus morhua L.) eggs may develop and hatch within temperatures of −1.5 to 12 °C, but little is known about the effects of very low temperatures on larval characteristics. Eggs of the Northeast Arctic cod (Gadus morhua) were incubated at 1, 5 or 8 °C from Day 1 after fertilisation until hatching, and transferred to 5 °C after hatching. Histological samples of the axial musculature were taken at hatching and 5 d after hatching, and the data on muscle cellularity from these samples were related to survival and hatching, size, developmental data and viability of the yolk sac larvae. All larvae hatched at the same developmental stage. Incubation of eggs at 1 °C produced shorter larvae with a larger yolk sac and more, small deep fibres at hatching than larvae from eggs incubated at 5 or 8 °C. The larval size difference was still present 5 d after hatching, a time at which the larvae from 1 °C-incubated eggs were less developed and less resistant to an acute viability stress test (65 ppt salinity). Although there were no differences between temperature groups in number and size of muscle fibres 5 d after hatching, the deep fibres of the 1 °C-group contained less myofibrils than the two other groups. The phenotype of the larvae at hatching was thus affected within these incubation temperatures. Although all groups were transferred to the same temperature after hatching, the lowest egg incubation temperature (1 °C) still had a negative effect 5 d after hatching, as these larvae were both smaller, less resistant to stress and had less functional muscles at the time of first feeding. Our conclusion is therefore that 1 °C is close to, or below, the lower thermal tolerance limit for normal functional development of Northeast Arctic cod. The results are discussed in relation to larval viability and recruitment of this species in the wild. Received: 4 February 1998 / Accepted: 10 July 1998