Juvenile Cod Gadus Morhua Research Articles

Sea cage aquaculture may provide an energetic subsidy to wild juvenile cod in coastal bays of southern Newfoundland, Canada

Sea cage aquaculture can alter the spatial distribution of wild fish populations; however, little is known about the dietary habits of wild fish frequenting sea cages. We used wild juvenile cod Gadus morhua reared in the laboratory and fed either an aquafeed pellet or marine-based diet to determine trophic discrimination factors (TDFs) of stable carbon (δ13C) and nitrogen (δ15N) isotope values in white muscle tissue and baseline liver proportions of vegetable-oil based (VOB) fatty acids. We then used δ13C and δ15N and proportions of VOB fatty acids to investigate the dietary habits of wild cage-associated juvenile and adult Atlantic cod and adult Atlantic redfish Sebastes fasciatus. Cod and redfish were collected in the immediate area of sea cages and reference areas of no aquaculture production. Juvenile cod captured around sea cages had both elevated δ13C and δ15N values and proportions of VOB fatty acids, and isotopic fractionation comparable to laboratory cod fed an aquafeed diet. However, differences in δ13C and δ15N and proportions of VOB fatty acids between cage-associated and reference site adult cod and redfish were predominately absent. Results suggest that sea cages may provide an energetic subsidy to juvenile cod but perhaps not at the level to sustain adult cod or redfish. Therefore, the lack of differences suggests that both adult groups may be using cage sites opportunistically and only for short duration, as isotopic values and fatty acid proportions were not consistent with waste feed consumption despite individuals being collected in close proximity to sea cages.

Settlement processes induce differences in daily growth rates between two co-existing ecotypes of juvenile cod Gadus morhua

Co-existence of sympatric populations of a fish species is a common phenomenon. In the fjords of southern Norway, 2 ecotypes of Atlantic cod Gadus morhua co-exist during their juvenile life stage. Cod of the North Sea ecotype are on average 2 cm larger than the fjord ecotype at the end of their first growing season in October, suggesting either differences in spawning time or differences in growth during the larval and early juvenile phase. We analysed 24352 daily otolith increments from 145 cod sampled in September 2000, 2003, 2008 and 2015 at 2 locations to estimate individual hatching times and daily growth rates. There was no significant difference in hatching time between ecotypes, locations or years. Population-specific models of the effect of year, temperature and fish length on growth rates showed that the North Sea ecotype grew significantly faster than the fjord ecotype, but growth rate differences were small until the juveniles reached ~40 mm and disappeared again in juveniles larger than 110 mm. The size range (40-110 mm) corresponds to the period following settling. This study documents how vital rates (e.g. growth rates) may change rapidly at ontogenetic transition points, leading to different phenotypic trajectories in co-existing ecotypes. It also highlights the importance of settlement as a key life-history transition that may amplify small existing differences in growth rates through positive feedback of size on competitive ability and eventually recruitment.

Feed Efficiency Versus Feed Conversion Ratio -- Demonstrated on Feeding Experiments with Juvenile Cod (Gadus Morhua)

Feed conversion ratio (FCR), which is used as a description of growth as a function of feed intake, depends mainly on the feeding rate but also on temperature. The estimation of the FCR is simple; however, the practical application of this ratio is limited to cases in which fish are fed beyond maintenance rate. In addition, FCRs are not comparable if they are measured at different temperatures. Ivlev (1939) proposed the more universal temperature independent metric K3 -- also known as feed efficiency. Even if the feed efficiency is a measure of the usable percentage of the food it has not been in common use in research and practice. This is most likely due to difficulties in determining this parameter in conjunction with temperature dependency of the maintenance rate and its temperature constant in practice. In this work we present an estimation method, which allows the determination of these parameters indirectly by the “system response” of fish. The parameter estimation is performed by least square fitting, comparing the food intake of the model with the food intake observed in feeding experiments with juvenile cod (Gadus morhua), carried out by Soofiani (1983) and Hawkins et al. (1985) over a wide range of feeding rates and temperatures. For individual fish, the model works not very accurately, while a comparison of the computed model results with the de facto consumed amounts (mean feed intake per fish and size group) demonstrates a high level of agreement. The relative estimation error is generally highest at the lowest consumption rates. The developed model is particularly suitable for the optimization of the feed ration at different temperatures in aquaculture systems.

Effects of small-scale habitat fragmentation on predator–prey interactions in a temperate sea grass system

During the last decades, fragmentation has become an important issue in ecological research. Habitat fragmentation operates on spatial scales ranging over several magnitudes from patches to landscapes. We focus on small-scale fragmentation effects relevant to animal foraging decision making that could ultimately generate distribution patterns. In a controlled experimental environment, we tested small-scale fragmentation effects in artificial sea grass on the feeding behaviour of juvenile cod (Gadus morhua). Moreover, we examined the influence of fragmentation on the distribution of one of the juvenile cod’s main prey resources, the grass shrimp (Palaemon elegans), in association with three levels of risk provided by cod (no cod, cod chemical cues and actively foraging cod). Time spent by cod within sea grass was lower in fragmented landscapes, but total shrimp consumption was not affected. Shrimp utilised vegetation to a greater extent in fragmented treatments in combination with active predation. We suggest that shrimp choose between sand and vegetation habitats to minimize risk of predation according to cod habitat-specific foraging capacities, while cod aim to maximize prey-dependent foraging rates, generating a habitat-choice game between predator and prey. Moreover, aggregating behaviour in grass shrimp was only found in treatments with active predation. Hence, we argue that both aggregation and vegetation use are anti-predator defence strategies applied by shrimp. We therefore stress the importance of considering small-scale behavioural mechanisms when evaluating consequences from habitat fragmentation on trophic processes in coastal environments.

Changes in pH during transport of juvenile cod Gadus morhua L. and stabilisation using buffering agents

Salmonids have been transported from hatchery to ongrowing units for many years, but little information is available on optimal conditions for the transport of marine finfish species. This study examined the changes in pH that occur under various simulated transport conditions of stocking density and fish size, and thereby recommends protocols for live marine fish transportation. The pH of the transport water declined rapidly due to the respiration of carbon dioxide. Various buffers (sodium bicarbonate, TRIS, Magnaspheres) were compared and maintained pH close to normal seawater levels. The decline in pH was greater with small (18g) rather than larger (32g) fish, at identical loading biomass in tanks, indicating that fish size should be considered when planning transport. Although unionised ammonia level increased with increasing stocking density (10 to 30kgm−3) the pH did not decline significantly. The use of air aeration as well as oxygenation reduced CO2 levels indicating that aeration stripped the CO2 from the water. Non invasive measurements of free cortisol release into the tank water by juvenile cod indicated primary stress response to handling events. This study demonstrates that cod juveniles will be exposed to fluctuating pH in transport tanks, and buffering of the pH and stripping of CO2 by aeration to reduce fluctuations is recommended.

Diet and feeding niches of juvenile Gadus morhua, Melanogrammus aeglefinus and Merlangius merlangus during the settlement transition in the northern North Sea

A study on the feeding ecology of juvenile cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus during the pelagic to demersal transition was carried out on fishes sampled throughout their settlement season at a local nursery ground in the north-western North Sea, off the Scottish east coast. A comprehensive quantitative taxonomic analysis of the diets, as described in the paper, showed the emergence of distinctive feeding niches, minimizing the potential for competition between species and size categories. The diet of the juveniles changed with fish size, water depth, time of year and distance offshore. Small G. morhua were present in the study area earlier in the season, settled further inshore and ate a higher proportion of pelagic prey (copepods) and as size increased they moved into deeper waters and targeted larger, more benthic prey. As M. aeglefinus grew larger and moved into deeper waters, a diet of largely copepods, amphipods, pelagic Ammodytes spp., cyprids and pelagic gastropods evolved to one dominated predominantly by fishes and benthic invertebrates. In the case of M. merlangus, widespread ages and sizes throughout the sampling season, a consequence of their more protracted spawning season, resulted in dietary changes which were more likely to be influenced by seasonal changes in the prey field, in addition to developmental (size) changes, than the diets of the other two species.

Enrichment promotes learning in fish

Captive birds and mammals reared in enriched rearing environments have been shown to behave more flexibly compared to animals reared in impoverished or plain environments. Recent evidence has shown that this is also true for fish; enrichment promotes faster recovery after a stress- ful experience, a higher propensity for exploration of novel areas and the development of more sophisticated social behaviour. Here we report how enrichment influences social learning in juvenile cod Gadus morhua that were reared in either spatially enriched or plain tanks. Naive juvenile cod were allowed to repeatedly observe experienced tutors as they foraged on gammarid or mysid prey, or control tutors that acted as social stimuli but did not forage. The naive fish then received a mixture of mysid and gammarid prey. Enriched-reared fish improved their ability to consume live prey in the presence of foraging tutors, but plain-reared fish did not. Although gammarids were consumed more often and more quickly than mysids, both among tutors and naive fish, social learning from tutors demonstrating mysid hunting and consumption had its greatest effect on social learning in the enriched fish.

Possible countergradient variation in growth of juvenile cod Gadus morhua from the northwest Atlantic

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 375:229-238 (2009) - DOI: https://doi.org/10.3354/meps07760 Possible countergradient variation in growth of juvenile cod Gadus morhua from the northwest Atlantic Manjusri P. Wijekoon1, Velmurugu Puvanendran2,*, Danny W. Ings1, Joseph A. Brown1,† 1Ocean Sciences Centre, Memorial University of Newfoundland, St. John’s, Newfoundland A1C 5S7, Canada 2Nofima, Muninbakken 9-13, 9291 Tromsø, Norway *Corresponding author. Email: velpuvy@nofima.no †Deceased ABSTRACT: To test the countergradient variation (CnGV) hypothesis and determine any genetic differences among populations, we conducted an experiment with juvenile cod originating from 3 regions in the NW Atlantic, 3Ps (Placentia Bay, Newfoundland, Canada; 48°N, 54°W), 4T (southern Gulf of St. Lawrence; 46°N, 61°W), and 4X (44°N, 67°W), at 2 temperatures (7 and 11°C) for 15 wk. Results indicated that temperature influenced the growth rates of individuals in all 3 populations, and juveniles reared at 11°C were significantly bigger (length and weight) than those reared at 7°C. No significant differences in growth and food conversion efficiency were found among the 3 populations. Although 3Ps juveniles were significantly smaller in length and lighter in weight than 4T and 4X juveniles at the start of the experiment, length and weight did not differ among populations at the end of the experiment. Survival of both 3Ps and 4T juveniles was significantly higher than that of 4X cod in both high and low water temperature treatments. Although statistical analyses showed no significant differences in growth and food conversion efficiency among juveniles from the 3 populations, reaction norm analysis showed a strong genotypic influence on growth and survival (no genotype × environment interactions) but a genotypic × environment interaction in food conversion efficiency. Our results suggest that Atlantic cod juveniles from these 3 regions do not show CnGV in growth and food conversion efficiency. However, performance differences among populations may be stage specific, and this may have masked CnGV in traits among Atlantic cod populations in our study. KEY WORDS: Growth · Geographic variation · Survival · Adaptation · Atlantic cod Full text in pdf format PreviousNextCite this article as: Wijekoon MP, Puvanendran V, Ings DW, Brown JA, † (2009) Possible countergradient variation in growth of juvenile cod Gadus morhua from the northwest Atlantic. Mar Ecol Prog Ser 375:229-238. https://doi.org/10.3354/meps07760 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 375. Online publication date: January 26, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Assessment of the diets of cod Gadus morhua and whiting Merlangius merlangus juveniles in a frontal region close to the Norwegian Trench: co‐existence or competition?

Over 200 pelagic juvenile cod Gadus morhua and whiting Merlangius merlangus were sampled in spring 2001 at stations inside and outside a frontal region close to the Norwegian Trench, and their stomach contents analysed in the context of zooplankton samples taken at the same time and same locations. Merlangius merlangus juveniles appeared to feed on a diverse zooplankton community, whereas G. morhua juveniles appeared to preferentially select Pseudocalanus elongatus, even though these prey items were not the dominant component of the prey community. The results suggest that during the pelagic juvenile stage, M. merlangus and G. morhua are direct competitors for zooplanktonic prey. In addition, because M. merlangus are more opportunistic, they may be more tolerant to changes in the species composition of the prey community, and therefore out‐compete G. morhua juveniles at times of low preferred prey abundance or under changing environmental conditions.

Escape responses in juvenile Atlantic cod Gadus morhua L.: the effects of turbidity and predator speed

We examined the effect of turbidity (0.5-14 beam attenuation m(-1)) and predator attack speed (150 and 296 cm s(-1)) on escape responses of juvenile cod Gadus morhua in the laboratory. We triggered escape responses using a predator model and measured escape timing, direction and locomotor performance. We also measured responsiveness and estimated the likelihood of fish escaping the ;predator attack' (putative escape success, PES). Turbidity affected both PES and the type of escape response used by the fish, but these effects depended on predator speed. PES for the fast predator attack declined from 73% in clear water to 21% in highly turbid water, due to decreased responsiveness and poorly timed escapes. Intermediate turbidity enhanced PES and responsiveness to the slow predator attack. Locomotor performance was reduced by turbidity, whereas predator speed had the opposite effect. Our results suggest that both predator attack speed and turbidity have important roles in determining the vulnerability of fish attacked by piscivorous predators.

Progress in determining southern blue whiting (Micromesistius australis) target strength: results of swimbladder modelling

Abstract Southern blue whiting target strength (TS) results from Kirchhoff modelling of swimbladder casts scanned using a hand-held 3D laser scanner are presented. The data are compared with the relationship between TS and fish length used for New Zealand stock-assessment surveys; TS = 21.8 log10(fork length) − 72.8, at 38 kHz. This relationship has its origins in the relationship used for blue whiting (Micromesistius poutassou) in the northern hemisphere, and is based on measurements on juvenile cod (Gadus morhua). The results indicate that the blue whiting relationship is not appropriate for southern blue whiting, and suggest a much steeper slope, with TS = 38 log10(fork length) − 97, at 38 kHz. Sensitivity analyses indicate that further investigations of swimbladder tilt-angle distribution and swimbladder volume are unlikely to provide evidence to support the use of the blue whiting relationship for southern blue whiting.

Bacteria in the gut of juvenile cod Gadus morhua fed live feed enriched with four different commercial diets

AbstractAtlantic cod, Gadus morhua L., larvae were fed rotifers, Brachionus plicatilis and Artemia franciscana enriched on four different commercial media, using the manufacturers' protocols. Pooled samples of 20 cod larvae were homogenized, diluted, and plated out on Petri dishes. The number of colony-forming units per larva was estimated, and the dominant strains subsequently sampled for sequencing of 16S rDNA. Bacteria showing high sequence similarity to a pathogen characteristic of cod and other fish species, Listonella anguillarum, were present in all four groups. Other taxa present among the dominating bacterial colonies were Pseudoalteromonas sp., and Vibrio sp. However, these bacteria could be assigned to genera only. The different enrichments probably affected the number of colony-forming bacteria per millilitre in the enrichment cultures as well as in the larval gastrointestinal (GI) tract. Also, the composition of the microbiota associated with the larval GI tract was probably affected by the enrichment media.

Influence of cruising and ambush predators on 3-dimensional habitat use in age 0 juvenile Atlantic cod Gadus morhua

Predators and prey often co-exist at high densities within the same habitat, yet the behavioural and spatial dynamics underlying this co-existence are not well known. To better understand small-scale, predator–prey co-occurrence, the spatial patterns and behaviour of age 0 juvenile cod Gadus morhua 75–88 mm SL and two of their known predators, age 2+ cod and short-horn sculpin Myoxocephalus scorpinus, were examined in two habitats (i.e., sand and eelgrass) using three-dimensional video analysis. Both habitat and predator type interacted to result in unique spatial patterns of prey. Spatial overlap between predators and prey was highest in open habitat in the presence of the cruising predator but lowest in the presence of sculpin in the same habitat. In eelgrass, age 0 cod avoided predators primarily along the vertical axis (i.e., distance off bottom). Age 0 cod stayed above eelgrass in the presence of sculpin but lowered themselves into the eelgrass while in the presence of predator cod. Anti-predator behaviour (i.e., predator–prey distance, prey cohesion and freezing) was significantly reduced over eelgrass compared to sand, suggesting eelgrass has lower ‘inherent risk’ than open habitats. However, predator consumption was similar across all treatments, suggesting that, 1) complex habitat also impairs the visual cues needed for anti-predator behaviour (e.g., schooling) and assessing the location of predators, and 2) predators change their behaviour with habitat to enhance their opportunities for finding and capturing prey.

Recovery from handling stress in Gadus morhua

The time needed for juvenile cod Gadus morhua to recover from handling stress, and also whether observer presence or haemoglobin genotype influenced recovery time, were investigated. Time to recovery after handling was found to be 1·5 h based on opercular beat frequencies and behaviour. No effects of haemoglobin genotype or observer presence were found.

Behavioural consequences of density-dependent habitat use in juvenile cod Gadus morhua and G. ogac: the role of movement and aggregation

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 272:257-270 (2004) - doi:10.3354/meps272257 Behavioural consequences of density-dependent habitat use in juvenile cod Gadus morhua and G. ogac: the role of movement and aggregation B. J. Laurel1,*, R. S. Gregory1,2, J. A. Brown1, J. K. Hancock1, D. C. Schneider1 1Memorial University of Newfoundland, Ocean Sciences Centre, St. John¹s, Newfoundland A1C 5S7, Canada 2Department of Fisheries and Oceans, St. John¹s, Newfoundland A1C 5X1, Canada *Email: blaurel@mun.ca ABSTRACT: Fish behaviour can change to accommodate a variable environment, but changes in behaviour have not been considered in the context of density-dependent habitat use. In Bonavista Bay, Newfoundland, we measured how fish density movement and schooling behaviour changed with habitat in 2 gadids, Atlantic cod Gadus morhua and Greenland cod Gadus ogac, using a combination of field-seines, mark-recapture and laboratory experiments. Density estimates from seines (n = 427) over 5 yr (1996, 1998 to 2001) indicated that these species associated with eelgrass but periodically were detected in high abundance over unvegetated habitat (i.e. sand). Within-site catch variation indicated both species aggregated more in sand than eelgrass habitat, and, in Atlantic cod, aggregations over sand increased as density in eelgrass increased. Although such patterns in catch data could be interpreted as due to the effects of differential mortality between habitats, a mark-recapture experiment indicated that both species of cod were not site-attached and moved between seine locations. Furthermore, video-analysis from laboratory experiments demonstrated that cod formed tighter aggregations over sand compared to eelgrass habitats. Our results suggest that juvenile cod modify their behaviour with changing density, possibly as a means of exploiting poor-quality habitats when high-quality habitats are saturated with conspecifics. Consequently, habitat suitability for Atlantic cod may be a dynamic rather than fixed variable in density-dependent habitat models. KEY WORDS: Density-dependence · Habitat selection · Atlantic cod · Greenland cod · Aggregation · Shoaling Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 272. Online publication date: May 19, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Trawl hangs, baby fish, and closed areas: a win–win scenario

Abstract The frequency and geographic distribution of trawlnet hangs from a fishery-independent survey are evaluated. The hangs data were plotted on a substratum map to confirm that many, but not all, were naturally occurring, high relief substrata. The data were also coupled with the occurrence of juvenile cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) to assess the degree of association between juvenile gadoids and high relief substrata. The average minimal distance from a fish occurrence to a hang ranged from 8.1 to 12.0 km (4.4–6.5 nautical miles), well within the reported daily range of movement for these fish. A similar pattern was detected for the sea raven (Hemitripterus americanus), a predator of juvenile gadoids, confirming the location of these microhabitat foodwebs. On average, closing an area 3.7 km (2 nautical miles) around a hang will enclose 17–30% of the populations of these juvenile fish; a wider buffer (18–28 km; 10–15 nautical miles) will close a linearly increasing portion of the populations. Additionally, closing areas surrounding the hangs, particularly regions of high hang density, will help to minimize losses of or damage to fishing gear. We propose a win–win scenario by establishing or evaluating closed areas in regions with high concentrations of known hangs. This approach is widely applicable for many marine ecosystems and may help to achieve simultaneous conservation and resource management goals, whereby one can both protect pre-recruit fish and enhance the effectiveness of a fishery.

Optimal selection of temperature areas by juvenile cod (Gadus morhua L.) in the Barents Sea modelled by dynamic optimisation

The trade-off between growth rate in areas with different temperatures and predation rates was modelled for juvenile cod living in the Barents Sea by a dynamic optimisation model. The choice of optimal temperature area of a juvenile (1-group) cod growing from a start weight (50 g, 15 cm) to a size refuge (640 g, 40 cm) was modelled for two scenarios with distributions of predators (4+ groups cod) similar to those observed in the Barents Sea in February 1990 and 1994, respectively, and without food limitations. The model predicted that when there was an overlap in distribution between large cod and younger cod (0–3 group), the younger year classes would move to areas with lower temperature and less predators, offering higher survival rates. Fish predators forced the prey to inhabit less optimal areas, and both growth and survival rates fell in comparison with a scenario with fewer fish predators (cannibals) and less overlap. Increased fish predator density increased these effects, and the juveniles would stay in colder water for a longer period and their growth rate would be further reduced. Higher mortality, independent of area and size, made it more profitable to stay in warmer water, leading to higher growth but increased mortality rates. 2001 International Council for the Exploration of the Sea

Stock‐specific changes in growth rates, food conversion efficiencies, and energy allocation in response to temperature change in juvenile Atlantic cod

Under common environments, populations of laboratory reared (Grand Banks, GB and Gulf of Maine, GOM) and wild caught (Fortune Bay, FB and Bonavista Bay, BB; Newfoundland) juvenile cod Gadus morhua responded similarly to temperature change in specific growth rates, food conversion efficiencies, condition factors, liver water content, and muscle water content. However, GOM cod had higher condition factors, and showed differences from GB cod in phenotypic plasticity of hepatosomatic index to temperature. These differences were not present in a different population comparison between FB and BB cod. All populations had higher growth rates and food conversion efficiencies at warmer temperatures, and exhibited compensatory growth when temperature was increased. The results suggest relatively larger genetic differences between GB and GOM cod than between FB and BB cod, and indicate that the faster growth of southern populations in the wild is not due to a higher genetic capacity for growth rate.

Spawning origins of pelagic juvenile cod Gadus morhua inferred from spatially explicit age distributions: potential influences on year-class strength and recruitment

Incongruity between the reconstructed spawning period of pelagic juvenile (0-group) cod Gadus morhua and the observed spawning period for adult cod on the main spawning grounds south of Iceland have indicated that large numbers of the surviving juvenile population may, in some years, originate from other smaller inshore spawning grounds located within fjords of the west, north and east coasts. We examine this hypothesis by using age/length and temperature/growth relation- ships, based on aging of 0-group cod from 1995 and 1997, to analyze historical demographic data from 0-group fish surveys (1970 to 1998) and determine the spawning distribution and origin of pelagic juvenile cod. Data on early life-history characteristics in conjunction with temporal and spa- tial distribution of pelagic juvenile cod are used to create new abundance indices representing the inshore and offshore juvenile cod-population components. Partitioning of the historical 0-group index into these population components at a finer geographic scale than has been used previously provides new information for estimating the relative contribution of the main spawning component versus the smaller in-fjord spawning components towards variable recruitment levels in the Icelandic cod stock. Spawning diversity, or multiple spawning components, are important for recruitment success as they disperse the mortality risk of the early life-history stages, which are thought to be the principal deter- minants of year-class strength.

Distinguishing small juvenile Atlantic cod ( Gadus morhua) from Greenland cod ( Gadusogac) by comparing meristic characters and discriminant function analyses of morphometric data

Distinguishing small juvenile Atlantic cod ( Gadus morhua) from Greenland cod ( Gadusogac) by comparing meristic characters and discriminant function analyses of morphometric data