Evaluating spatio‐temporal variability in the habitat quality of Atlantic cod (Gadus morhua) in the Gulf of Maine

Reduced abundance and contracted spatial distribution of Atlantic cod, Gadus morhua, in the Gulf of Maine (GOM) may indicate large spatio‐temporal variation in their habitat quality. Season‐specific Habitat Suitability Index (HSI) models were developed to quantify such variation in the offshore GOM management area. Data used were non‐zero cod catch rates with calibrations from the Northeast Fisheries Science Center (NEFSC) spring and fall bottom trawl surveys over the period 1982–2013 and key physical environmental variables including depth, bottom temperature, bottom salinity and sediment types. Significant declines were found in the average HSI across the study area in the springs of early 2000s and 2010s. These low average HSI values coincide with reduced age‐1 recruitment of GOM cod stock after the mid‐1990s. Moreover, the western coastal areas of the GOM generally exhibited higher average HSI values than the eastern coastal areas, whereas the offshore areas always had the lowest average HSI. Relatively higher cod survey catch rates in the western GOM may imply positive influences of environmental controls on the distribution of GOM cod.     

Influence of larval transport and temperature on recruitment dynamics of North Sea cod (Gadus morhua) across spatial scales of observation

The survival of fish eggs and larvae, and therefore recruitment success, can be critically affected by transport in ocean currents. Combining a model of early‐life stage dispersal with statistical stock–recruitment models, we investigated the role of larval transport for recruitment variability across spatial scales for the population complex of North Sea cod (Gadus morhua). By using a coupled physical–biological model, we estimated the egg and larval transport over a 44‐year period. The oceanographic component of the model, capable of capturing the interannual variability of temperature and ocean current patterns, was coupled to the biological component, an individual‐based model (IBM) that simulated the cod eggs and larvae development and mortality. This study proposes a novel method to account for larval transport and success in stock–recruitment models: weighting the spawning stock biomass by retention rate and, in the case of multiple populations, their connectivity. Our method provides an estimate of the stock biomass contributing to recruitment and the effect of larval transport on recruitment variability. Our results indicate an effect, albeit small, in some populations at the local level. Including transport anomaly as an environmental covariate in traditional stock–recruitment models in turn captures recruitment variability at larger scales. Our study aims to quantify the role of larval transport for recruitment across spatial scales, and disentangle the roles of temperature and larval transport on effective connectivity between populations, thus informing about the potential impacts of climate change on the cod population structure in the North Sea.     

Geolocation of Atlantic cod (Gadus morhua) movements in the Gulf of Maine using tidal information

Information derived from archival tags (digital storage tags, DSTs) were used to backtrack the migration of 11 tagged Atlantic cod (Gadus morhua) during 2001 in Massachusetts Bay, the Gulf of Maine, and Georges Bank. The DST tags continuously recorded time, temperature and depth. To geolocate fish positions during its time at large, we first extracted the tidal signal from the pressure recordings on the DST tags, and then compared the resulting data to data predicted with a Massachusetts Bay tidal model that provided us with geographical coordinates at a given date and time. Using least‐squares criteria within an estimated geographical region of confidence that was constrained by biological and statistical information (e.g. swimming speed, known release and recapture location, and bottom depth) we were able to geolocate the fish. The resultant geolocated migration tracks indicate a large degree of movement of Atlantic cod in the region and an elevated importance of the Great South Channel (GSC) migration corridor between Massachusetts Bay and the western Georges Bank and Nantucket Shoals region. This observation contrasts strongly with inferences of limited movements by Atlantic cod based on conventional tag recapture methods (mean of 1200 km traveled versus 44 km traveled as measured by conventional tagging and geolocation, respectively). This study demonstrates that geolocation methodologies applied to archival tag studies hold great promise of becoming an important new tool for fisheries managers to quantify the movements of fishes. It also points out the need for greater collaboration between fisheries scientists and oceanographers, and particularly for the development of improved tidal models to cover stock regions more accurately and with higher precision.     

Juvenile cod (Gadus morhua) mortality and the importance of bottom sediment type to recruitment on Georges Bank

The demersal settlement of pelagic juvenile fish has been considered a critical period in which the final adjustment is made to the size of a year class. Distribution patterns of pelagic and recently settled juvenile cod (Gadus morhua) were examined from nine surveys on Georges Bank during the summer over 5 years, 1984–1989, to relate juvenile survival to the sedimentary environment. Pelagic juveniles were widespread across Georges Bank in June, and by mid‐July they occurred on all bottom types from sand to gravel on eastern Georges Bank. However, by late July‐early August they were mostly abundant on the northeastern edge gravel deposit, which with its complex relief, provides abundant prey and refuge from predators. A bank‐wide estimate of pelagic juvenile abundance in 1986 and 1987 was used to assess mortality of the recently settled juveniles and to evaluate the relative importance of survivors from the northeastern edge gravel area to recruitment of the Georges Bank population. Settlement mortality rates over 1–2 months on the northeastern gravel area ranged from 3 to 8% day^?1, which compared reasonably with other studies. The seasonal abundance of the pelagic juveniles was almost an order of magnitude higher in 1987 than 1986; however, recruitment at age 1 was similar, indicating that a high mortality of the demersal juveniles occurred in 1987. The limited northeastern gravel area on Georges Bank may represent a survival bottleneck depending on the variability in the distribution and abundance of juvenile cod settlement in relation to that of their predators.     

Incorporating an environmental stock–recruitment relationship in the assessment of Pacific cod (Gadus macrocephalus)

The robustness of a previously described environmentally mediated stock–recruitment relationship for Pacific cod in Hecate Strait, BC, Canada was tested with 10 yr of additional data. The original analysis tested several alternative hypotheses and concluded that water transport through Hecate Strait, as indicated by sea level height, coupled with cod spawning biomass formed the best model. The present analysis indicates the relationship held through the 1990s. The implications of variation in sea level on stock production were investigated with a delay‐difference stock production model that included an environmentally mediated stock–recruitment relationship. The model predicted that the maximum fishery yield would vary between 1750 and 3670 t yr^?1 over the observed range of sea level height, and the estimated unsustainable fishing mortality during periods of low productivity would be only 0.5 times that in periods of high productivity.     

Re‐evaluating the effect of wind on recruitment in Gulf of Maine Atlantic Cod (Gadus morhua) using an environmentally‐explicit stock recruitment model

A previous study documented a correlation between Atlantic Cod (Gadus morhua) recruitment in the Gulf of Maine and an annual index of the north component of May winds. This correlation was supported by modeling studies that indicated strong recruitment of Gulf of Maine Atlantic Cod results from high retention of spring‐spawned larvae in years when winds were predominately out of the north, which favor downwelling. We re‐evaluated this relationship using updated recruitment estimates and found that the correlation decreased between recruitment and wind. The original relationship was largely driven by two recruitment estimates, one of which (2005 year‐class) was highly uncertain because it was near the terminal year of the assessment. With additional data, the updated assessment estimated lower recruitment for the 2005 year‐class, which consequently lowered the correlation between recruitment and wind. We then investigated whether an environmentally‐explicit stock recruit function that incorporated an annual wind index was supported by either the original or updated assessment output. Although incorporation of the annual wind index produced a better fitting model, the uncertainty in the estimated parameters and the implied unexploited conditions were not appropriate for providing management advice. These results suggest the need for caution in the development of environmentally‐explicit stock recruitment relationships, in particular when basing relationships and hypotheses on recruitment estimates from the terminal years of stock assessment models. More broadly, this study highlights a number of sources of uncertainty that should be considered when analyzes are performed on the output of stock assessment models.     

Residency of adult Atlantic cod (Gadus morhua) in the western Gulf of Maine

A mark and recapture study of cod in the western Gulf of Maine was conducted to study the seasonal movements of fish, particularly as they related to areas closed to commercial fishing. A total of 27,772 cod were tagged, and 1334 (4.8%) were recaptured with sufficiently detailed recapture location and date to be included in the study. Results indicated that the group is resident to the area and sedentary. Although there were a small percentage of fish (2.5%) that traveled long distances (>100 km), most were recaptured near their release location. There was no linear relationship between fish length and linear distances traveled, and linear distance traveled was only weakly related to days-at-large. For all groups of cod tagged and released in particular areas and months, mean distances traveled were small (<65 km), rates of travel were slow (<0.2 km/day), and rates of group dispersion were <50 km²/day. There were no recognizable spatial or temporal patterns in the mean angles of travel for groups released in various month/area combinations, and the angular deviations associated with the mean angles were quite large. Although there seems to be little pattern in the movement of cod in our study area, temporal and spatial changes in abundance indicate that movements are occurring.

The general pattern was a concentration of large cod into one small, inshore area (Area 133) in both the spring and winter, and dispersion from this area in the ensuing months. Monthly percentages of ripe cod in Area 133 provide evidence for two spawning groups; a winter group that spawns from November through January, and a spring group that spawns from April through July. Thus it is likely that the observed spring and winter concentrations of fish in Area 133 were associated with spawning. Fish in the spring group were the largest encountered in the study. The study also provided some evidence of natal homing, i.e. a return to the same spawning grounds year after year, for the spring spawning group. Although there is some exchange of fish between most of the closed areas, the timing of the closures appears to protect the largest aggregations of cod.     

Greenland cod (Gadus morhua): modeling recruitment variation during the second half of the 20th century

Two approaches were used to qualify observed variability in Greenland cod (Gadus morhua) recruitment. In the first analysis, we used the linear trend of the Greenland cod recruitment time series and climatic variables, such as air temperatures from the Denmark Strait and wind conditions off East Greenland and Southwest Greenland, to explain the interannual variation in cod recruitment off Greenland. The model resulting from this ‘trend/environmental approach’, explained 79% of the interannual variation in cod recruitment off Greenland. In the second, analytical approach, the ‘regime approach’, multiple linear regression models were used, with the input data being the time series of cod recruitment and spawning stock biomass (SSB) from Iceland and Greenland, sea surface and air temperatures around Greenland, and zonal wind components between Iceland and Greenland. Model results indicated that, during the decades between 1950 and 1990, there were three different cause–effect regimes which significantly influenced the variability of cod recruitment. The three regimes included: (a) the 1950s and 1960s, a regime with favorable sea surface temperatures and a self‐sustaining cod stock off Greenland with high SSB that produced a series of above‐average, strong year classes; (b) the 1970s and 1980s, a regime of declining SSB and recruitment, with recruitment dependent on advection from Iceland; and (c) the 1990s, when the advective potential for recruitment from the Icelandic cod stock was the only available source for replenishment of the Greenland cod stocks, because cod recruitment in Greenland waters was negligible. The three models explained 76–77% of the observed interannual variation in cod recruitment off Greenland. Both approaches suggested that advective factors were the dominant influences for cod recruitment in the ‘Iceland–Greenland System’.     

Modeled transport of winter flounder larvae spawned in coastal waters of the Gulf of Maine

Winter flounder, Pseudopleuronectes americanus, from southern New England and the Gulf of Maine were historically considered to be obligate estuarine spawners. However, recent experiments and observations document that winter flounder in the Gulf of Maine also utilize coastal waters for spawning. An individual‐based modeling approach was used to investigate the transport of winter flounder larvae from three hypothesized coastal spawning grounds in the Gulf of Maine. Transport success rates were greatest for larvae released from Ipswich Bay, intermediate for Stellwagen Bank and least successful for those released from Jeffreys Ledge. There was substantial interannual variability in larval transport and geographic patterns of potential connectivity. Furthermore, the date of spawning had an important influence on transport success. Model results suggest that certain coastal spawning grounds used by winter flounder may serve as an important source of larvae to estuaries and nearshore nursery areas. The potential influx of coastal spawned larvae could have implications for the resilience, productivity and gene flow in local populations. Model results provide further support for the conclusion that winter flounder in the Gulf of Maine may not be solely dependent upon estuaries for spawning. Results also suggest that coastal spawning groups should be considered explicitly in the management of winter flounder, and protected under Essential Fish Habitat regulations.     

Inter‐individual variation in the dominant intestinal microbiota of reared Atlantic cod (Gadus morhua L.) larvae

The diversity of the microbiota in 15 Atlantic cod (Gadus morhua L.) larvae from three different hatcheries with variable food type and water treatment was characterized with culture dependent and independent approaches. The culture‐dependent approach was phenotypic characterization of bacterial isolates based on 16 tests, and the culture‐independent approach was terminal restriction fragment length polymorphism (T‐RFLP) of PCR amplified 16S rRNA. Both approaches revealed large inter‐individual differences in the microbiota of cod larvae. Data from real time PCR indicate substantially lower variation in bacterial number larva^?1 both within and between hatcheries than the data on colony forming units, resulting in an estimated four orders of magnitude variation in culturability between larvae. Considerably higher richness and diversity were observed with T‐RFLP than with phenotypic characterization, and no correlation between the two approaches was found for diversity indexes calculated at the individual level. Dissimilarity in the composition of the microbiota between individuals from the same rearing environment was comparable to the dissimilarities between hatcheries. The large inter‐individual variation is important to consider when designing experiments, as pooling of individuals will completely mask this variability. We conclude that colonization of individual cod larvae by bacteria seems to be a stochastic and not a deterministic process, which is not well understood.     

Optimum temperature and food-limited growth of larval Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) on Georges Bank

We estimated recent growth of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae collected on the southern flank of Georges Bank in May 1992–94 from the ratio of RNA to DNA (R/D) and water temperature. Growth of both species increased with water temperature to about 7°C and then decreased. The highest growth rates were observed in May 1993 at water temperatures around 7°C. These data confirm an earlier observation of comparable temperature optima for growth of Atlantic cod and haddock larvae in the north‐west Atlantic. Comparisons of field growth rates and temperature optima with data for larvae cultured at high temperatures and prey densities in the laboratory suggest that growth may have been food‐limited at higher temperatures on Georges Bank. Given that 7°C is the long‐term mean water temperature on the southern flank in May and that climate models predict a possible 2–4°C rise in water temperatures for the western North Atlantic, our findings point to a possible adverse effect of global warming on Atlantic cod and haddock.     

Cryopreservation of Atlantic cod (Gadus morhua) sperm in large‐volume straws: applications for commercial production and gene banking

In our study, we used a full factorial analysis of variance design to examine the effects of diluent [Mounib's sucrose‐based diluent+hen's egg yolk (EY) and Hanks' balanced salt solution (HBSS)+EY], freezing rate (?2.5, ?5.0 and ?7.5 °C min^?1) and thawing rate (2.5, 5.0 and 7.5 °C min^?1) on motility and velocity of Atlantic cod sperm cryopreserved in 2.5 mL cryogenic straws. We found that post‐thaw sperm performance was strongly influenced by the presence of higher‐order interactions of the factors we tested. For all models broken down by diluent, the 2.5 °C min^?1 thawing rate had the lowest sperm motility recovery index. Mounib's sucrose‐based diluent+EY had the highest motility recovery index at all thawing rates. Mean per cent motility for fresh sperm (87.7±2.9%) was not significantly different than of sperm cryopreserved using Mounib's sucrose‐based diluent+EY, frozen at ?2.5 °C min^?1 and thawed at 5.0 °C min^?1 (77.1±2.9%). For Mounib's sucrose‐based diluent+EY, velocity was significantly higher with sperm thawed at 7.5 °C min^?1, than sperm thawed at 2.5 °C min^?1, while thawing rate had no effect for HBSS+EY. Our findings have implications for cod mariculture and aiding in conservation efforts for a dominant marine fish species.     

Technological solutions and operational measures to prevent escapes of Atlantic cod (Gadus morhua) from sea cages

Escapes of cod (Gadus morhua) from sea cages represent an economic problem for farmers and a potential environmental problem. We estimate that 0–6% of cod held in sea‐cage farms in Norway were reported to have escaped each year from 2000 to 2005, which is a high proportion compared with salmon. We interviewed employees at 19 coastal sea‐cage cod farms in Norway to investigate both how and why cultured cod escape and to document cage handling and management strategies that were effective in minimizing escapes. Based on the interviews, we describe five working hypotheses that may explain why a greater proportion of cod than salmon escape: (1) cod are more willing to escape than salmon; (2) cod bite the net cage and create wear and tear; (3) net cages have insufficient technical standards for cod culture; (4) cod are placed in sea cages at considerably smaller sizes than salmon; and (5) cod are more popular feed for predators. Preliminary testing of the hypothesis that cod bite netting and create holes was done by placing pre‐damaged net panels with cut twines and control panels inside sea cages. Holes in the pre‐damaged net panels increased in size over a period of 3 months. The type of damage indicated that biting of netting twines was the likely cause.     

Carnobacterium maltaromaticum vs. Vibrio (Listonella) anguillarum in the midgut of Atlantic cod (Gadus morhua L.): an ex vivo study

The present investigation addresses whether the midgut (MG) of Atlantic cod (Gadus morhua L.) is an infection route for Vibrio (Listonella) anguillarum serotype 02 β and if Carnobacterium maltaromaticum, a probiotic bacterium, can out‐compete the pathogen and modulate the autochthonous MG microbiota. This was investigated by using an ex vivo method the intestinal sac, utilized previously in studies on Atlantic cod and Atlantic salmon (Salmo salar L.). Exposure of the MG to V. (L.) anguillarum did not reveal any cell damage indicating that the MG does not appear to be an infection route for V. (L.) anguillarum in healthy Atlantic cod. This finding together with previous observations on Atlantic cod and Atlantic salmon indicate that intestine as an infection route might vary between these two species. When the MG was exposed to C. maltaromaticum, no cell damage or cellular disruptions were observed. As budding from the apices of microvilli was observed in all treatments exposed to bacteria, we suggest that budding might be involved in the primary barrier against bacterial infection. However, to clarify this hypothesis, further studies are needed. Exposure of the MG to the probiotics and pathogenic bacteria indicated that C. maltaromaticum, to some extent, is able to out‐compete V. (L.) anguillarum but the topic merits further investigation. Analysis of the MG microbiota after sterile saline solution and bacterial exposure indicates that bacteria related to Staphylococcus sciuri belong to the autochthonous gut microbiota in Atlantic cod.     

Effects of ontogeny, temperature, and light on vertical movements of larval Pacific cod (Gadus macrocephalus)

The role of behavior, especially vertical migration, is recognized as a critical component of realistic models of larval fish dispersion. Unfortunately, our understanding of these behaviors lags well behind our ability to construct three-dimensional flow-field models. Previous field studies of vertical behavior of larval Pacific cod ( Gadus macrocephalus ) were limited to small, preflexion stages (≤11 mm SL) in a narrow range of thermal conditions. To develop a more complete picture of larval behavior, we examined the effects of ontogeny, temperature, and light on vertical responses of larval Pacific cod in experimental columns. While eggs were strictly demersal, yolk-sac larvae displayed a strong surface orientation as early as 1 day post hatch (∼ 5 mm SL). Consistent with field observations, small preflexion larvae (<10 mm SL) showed no response to varying light levels. However, there was a direct effect of temperature on larval behavior: Pacific cod larvae exhibited a stronger surface orientation at 4°C than at 8°C. The behavior of larger, postflexion larvae (>15 mm SL) in experimental columns was consistent with a diel vertical migration and independent of water temperature: fish were more widely distributed in the column, and median positions were consistently deeper at higher light levels. These laboratory observations are combined with observations from discrete-depth (MOCNESS) sampling in the Gulf of Alaska to characterize the vertical distribution of larval Pacific cod and contrast ontogenetic patterns with walleye pollock ( Theragra chalcogramma ). The vertical movements of larval Pacific cod described here will be applied in the development of dispersal projections from Gulf of Alaska spawning grounds.     

Experimental study of the susceptibility of Atlantic cod, Gadus morhua (L.), to infection with an IPNV strain pathogenic for Atlantic salmon, Salmo salar L

Intraperitoneal (IP) injection, cohabitation and immersion routes of infection were used to determine if Atlantic cod, Gadus morhua (L.), of 1 and 3 g are susceptible to infectious pancreatic necrosis (IPN). Mortalities of cod injected IP were significantly higher when challenged with infectious pancreatic necrosis virus (IPNV) than with phosphate buffered saline. This is the first report of Atlantic cod mortalities caused by IPNV. Fish challenged by cohabitation had significantly higher mortalities than the controls, but mortalities of Atlantic cod challenged with IPNV by immersion were not significantly different from controls. Titres of IPNV in the tissues of infected fish were sometimes very high (range 10²–10¹⁰ infectious units per gram of tissue) suggesting virus replication and titres of fish that died were generally higher than those of fish which survived. However, the relatively low mortality rates when challenged by cohabitation and immersion (20% and 17%, respectively), compared to the IP injection challenge (100%) suggest that 1 and 3 g cod have low susceptibility to IPN when challenged by more natural routes. These data strongly suggest that the cause of death of experimentally challenged cod was IPNV and further histological evidence for this came from 1 g cod challenged IP with IPNV in which the pancreas showed severe necrosis and heavy immunostaining for IPNV coincidentally with the peak of mortalities.     

Development of a continuous cell line from larval Atlantic cod (Gadus morhua) and its use in the study of the microsporidian,Loma morhua

In vitro cell culture methods are crucial for the isolation, purification and mass propagation of intracellular pathogens of aquatic organisms. Cell culture infection models can yield insights into infection mechanisms, aid in developing methods for disease mitigation and prevention, and inform commercial‐scale cultivation approaches. This study details the establishment of a larval cell line (GML‐5) from the Atlantic cod (Gadus morhua) and its use in the study of microsporidia. GML‐5 has survived over 100 passages in 8 years of culture. The line remains active and viable between 8 and 21°C in Leibovitz‐15 (L‐15) media with 10% foetal bovine serum and exhibits a myofibroblast phenotype as indicated by immuno‐positive results for vimentin, α‐smooth muscle actin, collagen I and S‐100 proteins, while being desmin‐negative. GML‐5 supports the infection and development of two microsporidian parasites, an opportunistic generalist (Anncaliia algerae) and cod‐specific Loma morhua. Using GML‐5, spore germination and proliferation of L. morhua was found to require exposure to basic pH and cool incubation temperatures (8°C), in contrast to A. algerae, which required no cultural modifications. Loma morhua‐associated xenoma‐like structures were observed 2 weeks postexposure. This in vitro infection model may serve as a valuable tool for cod parasitology and aquaculture research.     

Spatiotemporal variability in the phenology of the initial intra‐annual molt of American lobster (Homarus americanus Milne Edwards, 1837) and its relationship with bottom temperatures in a changing Gulf of Maine

This study uses the Maine Department of Marine Resources Lobster Sea Sampling data (2000–2016) and logistic models to develop the first time series for the timing and suddenness of onset of the initial intra‐annual molt of American lobster in the Gulf of Maine (GoM), an annual fishery recruitment event crucial to fishermen. Data from three GoM regions (eastern, central, and western coastal Maine) were further divided by sex and estimated maturity of sampled lobsters for analysis. We found differences in the patterns of initial molt timing and suddenness between the regions, sexes, and stages of maturity. Using the Northeast Coastal Ocean Forecasting System hindcast temperatures, seasonal accumulated degrees above 5°C were used to describe the thermal history for each region at ocean depths of about 5 and 110 m. These temperature metrics were used in generalized linear models to investigate the potential effects of seasonal temperatures on the initial molt season. Results showed that initial intra‐annual molting of lobsters was variable from 2000 to 2016, with periods of both earlier and more sudden molts and later and more protracted molts. Warmer temperatures, specifically inshore temperatures, were generally associated with an earlier molt, but without complete uniformity in the direction and magnitude across seasons, regions, and lobster demographics. We also discuss why developing molt time series and quantifying the connection to the bottom temperatures are necessary and emphasized why existing monitoring programs and the applied quantification techniques herein make this relationship difficult to quantify.     

The influence of temperature on advective loss of Atlantic cod (Gadus morhua) eggs from the inshore environment

We use a simple model of temperature-dependent egg development and mortality to develop several hypotheses concerning the effect of temperature on the occurrence of eggs of Atlantic cod, Gadus morhua , in Placentia Bay, Newfoundland. Predictions from this exploratory model were tested through a series of synoptic ichthyoplankton surveys throughout spawning and postspawning periods during 1997 and 1998. Although several egg mortality relationships were explored through the simulation, a constant mortality rate best represented the pattern observed in the two years of data. Peaks in late stage egg densities occurred in August of both 1997 and 1998 and were apparently decoupled from egg production peaks in April. We observed a decrease in mortality and the distance dispersed during egg development with increases in water temperature. We suggest that the effects of predation are small relative to the advective effects within this system, and that the interaction between advection and temperature-dependent vital rates of eggs may have dramatic consequences for coastal retention of propagules produced by inshore spawning events.     

Effect of cultivated copepods (Acartia tonsa) in first‐feeding of Atlantic cod (Gadus morhua) and ballan wrasse (Labrus bergylta) larvae

The aim of this study was to compare the nutritional composition and effects of short periods with cultivated copepod nauplii versus rotifers in first‐feeding. Atlantic cod (Gadus morhua) and ballan wrasse (Labrus bergylta) larvae were given four different dietary regimes in the earliest start‐feeding period. One group was fed the copepod Acartia tonsa nauplii (Cop), a second fed enriched rotifers (RotMG), a third fed unenriched rotifers (RotChl) and a fourth copepods for the seven first days of feeding and enriched rotifers the rest of the period (Cop7). Cod larvae were fed Artemia sp. between 20 and 40 dph (days posthatching), and ballan wrasse between 36 and 40 dph, with weaning to a formulated diet thereafter. In addition to assessing growth and survival, response to handling stress was measured. This study showed that even short periods of feeding with cultivated copepod nauplii (7 days) had positive long‐term effects on the growth and viability of the fish larvae. At the end of both studies (60 days posthatching), fish larvae fed copepods showed higher survival, better growth and viability than larvae fed rotifers. This underlines the importance of early larval nutrition.