Wild Cod Research Articles

Comparative Study of Fatty Acid Composition of Muscles of Atlantic Cod (Gadus morhua Linnaeus, 1758) with Natural Diet and Feeding near Salmon Farms

Coastal aquaculture and local fisheries interact in shared marine environments, influencing each other synergistically and/or antagonistically. Salmon farming, notably with open-net sea cages along the Norwegian coast, attracts wild fish due to increased food availability from uneaten feed, but it also exposes wild fish to farm emissions like waste and toxic chemicals (de-lice treatments, antifouling and medical agents). The attraction behaviour of wild fish can impact fatty acid composition in fish tissues, influenced by the high terrestrial fat content in salmon aquafeed. We study how the Atlantic cod, aggregating around salmon farms in a subarctic fjord in Northern Norway, can be affected, potentially altering their natural diet and fatty acid profiles. Our study compares the muscle-tissue fatty acid compositions of cod caught near aquaculture facilities (impact) versus fish caught in neighbouring fjords (control), and we hypothesise decreased omega-3 fatty acids near farms. The analysis revealed no significant differences in the fatty acid concentrations or categories between the impacted and control fish, challenging our initial expectations. However, differences were found for C18:1(n9)t (elaidic acid), with a higher value in the impacted fish. These findings suggest that salmon farming’s influence on cod’s fatty acid profiles in the flesh (i.e., relevant for the nutritional quality of the fillets that consumers eat) may be limited or minimal despite their aggregative behaviours around farms. The threshold levels of salmon feed consumed by wild cod before it affects the quality and survival of, e.g., sperm or other life stages, are not known and require new investigations. This study underscores the complexity of interactions between aquaculture and wild fisheries, impacting both ecological dynamics and consumer perspectives on seafood quality and health benefits.

Genetic analyses verify sexually mature escaped farmed Atlantic cod and farmed cod eggs in the natural environment.

Elucidating the effects of domesticated organisms escaping into the natural environment represents a topic of importance in both evolutionary and conservation biology. However, when excluding the abundant data on salmonids, there is a lack of knowledge on this topic for marine fish aquaculture, which continues to expand globally. In order to bridge this empirical gap, we investigated a suspected escape of sexually mature domesticated Atlantic cod from a commercial marine fish farm in northern Norway. This involved genotyping samples of fish from cages on the farm, putatively identified escapees and wild cod captured in the region and samples of recently spawned eggs collected in the sea. Genetic analyses confirmed a farmed ancestry of the suspected escapees, and significantly, 27% of the sampled cod eggs. Furthermore, statistical analyses revealed a strong reduction in genetic variation in all samples of the farmed cod, including low effective population size and high degree of siblingship. These results thus document the escape of sexually mature adult cod and the release of fertilized domesticated cod eggs into the natural environment. Although it is possible that some of the mature escapees spawned post-escape, the fact that only a single egg of potential hybrid farmed × wild origin was identified, together with the high number of mature cod in the farm, points to within cage spawning as the primary source of these eggs. This suggestion is supported by oceanic particle-drift modelling, verifying that transport of eggs between the farm and the egg sampling locations was plausible. This study represents a rare documentation of interaction between domesticated and wild populations for a marine fish, pointing towards potential impacts on the local wild population.

Ecological interactions between farmed Atlantic salmon and wild Atlantic cod populations in Norway: A review of risk sources and knowledge gaps

AbstractAquaculture provides an important and expanding source of protein rich and healthy food to the world. However, to minimize environmental harm from aquaculture, interactions with wild fish communities need to be thoroughly assessed. Here, we characterize the existing knowledge pertaining to such interactions, exemplified with Atlantic salmon (Salmo salar) farming in open net pens along the Norwegian coast and potential consequences for wild Atlantic cod (Gadus morhua) populations. Importantly, the wild cod fishery also provides a protein rich, high quality food source with high economic value. We identify seven risk sources that may affect behaviour, physiology, and survival in wild cod. Of particular importance is the large amount of waste feed that causes wild fish to aggregate around farms, thereby altering a multitude of ecological interactions including predation and disease transmission. Moreover, altered food quality in pellets may alter physiological processes and cause mortality to vulnerable life‐stages in wild cod. More research is needed on mechanisms and thresholds for harm. As the most important cod fisheries are found in northern Norway, where climate change also is rapid, we expect stronger and potentially more harmful interactions between fish farming and wild cod fisheries as aquaculture continues to expand. We hope that our analysis will inspire further research, on farmed salmon and wild cod interactions, but also on aquaculture and wild fish interactions in general. Such research is fundamental for the development of management systems that can reduce the impact of aquaculture on fisheries and the environment.

Bold-shy personality traits of globally invasive, native and hatchery-reared fish.

Bold behaviour of non-native species is hypothesized to facilitate invasion success, yet extreme boldness in wild and domesticated animals can be maladaptive. The purpose of this study was to compare individual behaviour among Australian native hatchery-reared (n = 33) and wild (n = 38) Murray cod (Maccullochella peelii) with invasive common carp (Cyprinus carpio; n = 30). Three laboratory tests measured individual behaviour: (1) emergence from a shelter, (2) exploration of a novel environment, and (3) approaching a predator. Wild invasive carp and hatchery-reared cod were generally faster and more likely to emerge and explore novel environments when compared with wild Murray cod. The 'bold-type' behaviours of hatchery-reared native cod were more like invasive carp than they were to 'shy-type' wild conspecifics, yet an important difference was that hatchery-reared cod spent substantially more time near a large predator while carp rapidly escaped. We suggest that these results are consistent with a bold-type invasion syndrome in invasive carp and learned boldness of hatchery-reared Murray cod. The propensity of invasive carp to rapidly explore and enter new environments, along with a fast predator escape response may have been important to their invasion success, while extreme risk-taking and predator naivety of hatchery-reared Murray cod may exacerbate post-release mortality rates in fisheries and conservation stocking programmes.

Terrestrial fatty acids from feed oil in feed for farmed salmonids are transferred to the liver, gonads, and muscle of wild Atlantic cod (Gadus morhua)

Abstract Wild fish attracted to salmon farms feed on waste feed that presently contain high levels of fatty acids of terrestrial origin. This study examines whether mature Atlantic cod (Gadus morhua) caught at spawning grounds has eaten salmon waste feed. Cod were caught at four spawning grounds around Smøla (Norway), an area with multiple salmon farms, during the spawning season in 2018 (n = 327) and 2019 (n = 488). The fatty acid (FA) profile of their livers, gonads (ovary and testis), and muscles (2019) were determined. Multivariate k-mean cluster analysis of liver FA profiles revealed three main clusters, which could be allocated to trophic niches using known fatty acid trophic markers (FATMs). Of the sampled cod in 2018 and 2019, 13 % and 20 % respectively had high liver concentrations of terrestrial FATMs (18:1n-9, 18:2n-6, and 18:3n-3), indicating waste feed feeding. The remaining cod could be assigned to either the pelagic or benthic food chain. The cod identified as feeding on waste feed had large, fatty livers. The terrestrial FAs were also transferred to the muscle and gonad lipids. It is postulated that the latter may result in gametes with sub-optimal lipid composition, potentially impacting fitness, which warrants further investigation.

High-frequency depth changes in Atlantic cod studied with implanted data storage tags

The main aim of this study was to investigate high-frequency depth changes in wild adult Atlantic cod Gadus morhua. The analysis was based on depth measurements collected with implanted data storage tags. The study was part of a ranching project carried out in an Icelandic fjord. In the project, net bags with frozen fish were regularly provided during the daytime at 4 stations where some cod formed distinct ‘herds’ (‘herd cod’) that did not mingle much with the rest of the unconditioned cod in the fjord (‘wild cod’). After tagging, some of the cod resumed life in the herds, whereas other cod left the herds immediately. On 20 subsequent Mondays, the electronic tags were programmed to measure at the highest frequency (every 30 s), and these results were used to study high-frequency depth changes in 4 wild cod and 4 herd cod, the latter as a control group. Several times, rapid cyclical depth changes were observed in both groups. This behaviour, which sometimes lasted for hours, was highest during dawn and dusk in wild cod but peaked during daytime in herd cod after deployment of the feed bags. The occurrence and properties of these vertical undulations varied greatly between fish, dates, and time of day. Most commonly, the periods of the cycles varied between 1 and 4 min and the heights between 2 and 4 m, but there were examples of much larger undulations. The results indicate that wild adult cod swim along vertically undulating paths when searching for prey, most likely to optimize foraging.

Effect of sea cage aquaculture on the length, weight, and condition of wild Atlantic cod Gadus morhua

Wild fish have been shown to associate with sea cage aquaculture and consume waste feed; however, little is known about the effects of waste feed consumption on wild fish. We used Atlantic cod Gadus morhua collected in the direct vicinity of sea cages and reference Northwest Atlantic Fisheries Organization divisions, either outside the direct vicinity (‘local division’) or completely removed from aquaculture (‘outside divisions’), to compare length, weight, and condition for cod ages 2 to 4. Concentrations of vegetable oil-based fatty acids (linoleic acid and α-linolenic acid) were then used as biomarkers for waste feed consumption to determine their role in explaining differences in length, weight, and Fulton’s condition index among cage-associated cod. Age 2 cage-associated cod were in lower condition than age 2 cod from the local division and lighter than age 2 cod from all outside divisions. Age 3 cage-associated cod were comparable to age 3 cod from the local division but in lower condition than age 3 cod from all outside divisions. However, age 4 cage-associated cod were longer and heavier than age 4 cod from the local division, but in lower condition than age 4 cod from 2 of 3 outside divisions. Additionally, there were positive relationships between fatty acid concentrations and length and weight for age 2 cage-associated cod, but no significant relationships for age 3 or 4 cage-associated cod. Results suggest the effects of waste feed consumption are contradictory but not consistent enough to disrupt established growth and condition patterns among divisions.

Broodstock exposure to warming and elevated pCO2 impairs gamete quality and narrows the temperature window of fertilisation in Atlantic cod.

Impacts of global warming and CO2 -related ocean acidification (OA) on fish reproduction may include chronic effects on gametogenesis and gamete quality, as well as acute effects on external fertilisation. Here, temperature thresholds and OA-sensitivity of gametogenesis and fertilisation were investigated in Atlantic cod, Gadus morhua. Three broodstock groups of farmed cod (FC 1-3) were exposed for 3 months to three maturation conditions (FC 1: control, 6°C/400 μatm CO2 ; FC 2: warming, 9.5°C/400 μatm; FC 3: warming and OA, 9.5°C/1100 μatm). In addition, a broodstock group of wild cod (WC) was kept at control conditions to compare the acute temperature window of fertilisation with that of farmed cod (FC 1). Fertilisations were conducted in a temperature-gradient table at 10 temperatures (between -1.5 and 12°C) and two CO2 levels (400/1100 μatm). In FC 1 and WC, fertilisation success was relatively high between 0.5°C and 11°C (TRange of c. 10.5°C), indicating similar gamete quality in farmed and wild broodstocks kept at control conditions. Exposure of farmed broodstocks to warming (FC 2) and the combination of warming and OA (FC 3) impaired gamete quality, causing a reduction in fertilisation success of -20% (FC 2) and - 42% (FC 3) compared to FC 1. The acute temperature window of fertilisation narrowed from FC 1 (TRange =10.4°C) to FC 2 (TRange =8.8°C) and FC 3 (TRange =5.9°C). Acute effects of CO2 on fertilisation success were not significant. This study demonstrates potential climate change impacts on gametogenesis and fertilisation in Atlantic cod, suggesting the loss of spawning habitat in the coming decades.

Absence of zoonotic parasites in salmonid aquaculture in Denmark: Causes and consequences

A parasitological investigation of Danish farmed freshwater (180 fish) and marine rainbow trout (170 fish), Oncorhynchus mykiss , was performed with an emphasis on the occurrence of zoonotic parasites. Reliability of the parasitological methods applied was assessed by examining fish from a stock of wild Atlantic cod Gadus morhua caught in the Sound (Øresund). While wild fish carried a number of zoonotic parasites, we found that farmed fish, both in the fresh and marine environment, were free of these parasites. Non-zoonotic trout parasites found were eye-flukes Diplostomum paracaudum, D. pseudospathaceum and Eubothrium crassum in maricultured rainbow trout. The acanthocephalan Echinorhynchus truttae and Diplostomum sp. were found in freshwater trout. In contrast, wild cod were infected with five different nematode species ( Anisakis simplex , Contracaecum osculatum , Pseudoterranova decipiens , Hysterothylacium aduncum , Cucullanus cirratus ), three trematode species ( Lepidapedon elongatum , Cryptocotyle lingua , Hemiurus communis ), one copepod species ( Lernaeocera branchialis ) and one myxozoan ( Myxobolus aeglefini ). The risk factors associated with aquaculture production in an infectious aquatic environment are analyzed and reasons for absence of zoonotic parasites in aquacultured trout are pinpointed. Based on the analysis recommendations for management practices securing parasite freedom in aquacultured fish are given. • Salmonid aquaculture in Denmark is free from zoonotic parasites. • Fish parasitological methods are validated by examination of wild fish. • Risk factors associated with aquaculture production are evaluated. • Recommendations for prevention of zoonotic parasite introduction are presented.

Accuracy of Otolith Oxygen Isotope Records Analyzed by SIMS as an Index of Temperature Exposure of Wild Icelandic Cod (Gadus morhua)

Global warming is increasing ocean temperatures, forcing marine organisms to respond to a suite of changing environmental conditions. The stable oxygen isotopic composition of otoliths is often used as an index of temperature exposure, but the accuracy of the resulting temperature reconstructions in wild, free-swimming Atlantic cod (Gadus morhua) has never been groundtruthed. Based on temperatures from data storage tags (DST) and corresponding salinity values, the stable oxygen isotope (δ18O) value was predicted for each month of tagging and compared with δ18Ootolith values measured in situ with secondary ion mass spectrometry (SIMS). Paired-sample Wilcoxon tests were applied to compare measured and predicted δ18O values. The difference between measured and predicted mean and maximum δ18Ootolith values was not significant, suggesting a good correspondence between SIMS-measured and DST-predicted δ18Ootolith values. However, SIMS-measured and predicted minimum δ18Ootolith values were significantly different (all samples: p < 0.01, coastal and frontal cod: p < 0.05), resulting in overestimation of maximum temperatures. Our results confirm that otoliths are well-suited as proxies for mean ambient temperature reconstructions. A possible matrix effect and the absence of a reliable aragonite standard for SIMS measurements appeared to cause a small divergence between measured and predicted δ18Ootolith values, which affected the estimation accuracy of absolute temperature. However, relative temperature changes were accurately estimated by SIMS-analyzed δ18Ootolith values.

Structural Identification of the Pacemaker Cells and Expression of Hyperpolarization-Activated Cyclic Nucleotide-Gated (HCN) Channels in the Heart of the Wild Atlantic Cod, Gadus morhua (Linnaeus, 1758).

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are proteins that contain highly conserved functional domains and sequence motifs that are correlated with their unique biophysical activities, to regulate cardiac pacemaker activity and synaptic transmission. These pacemaker proteins have been studied in mammalian species, but little is known now about their heart distribution in lower vertebrates and c-AMP modulation. Here, we characterized the pacemaker system in the heart of the wild Atlantic cod (Gadus morhua), with respect to primary pacemaker molecular markers. Special focus is given to the structural, ultrastructural and molecular characterization of the pacemaker domain, through the expression of HCN channel genes and the immunohistochemistry of HCN isoforms, including the location of intracardiac neurons that are adjacent to the sinoatrial region of the heart. Similarly to zebrafish and mammals, these neurons are immunoreactive to ChAT, VAChT and nNOS. It has been shown that cardiac pacemaking can be modulated by sympathetic and parasympathetic pathways, and the existence of intracardiac neurons projecting back to the central nervous system provide a plausible link between them.

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.

Age validation of age 0-3 wild cod Gadus morhua in the western Baltic Sea through mark-recapture and tetracycline marking of otoliths

The contrasting zones within otoliths are commonly used to infer age of fish, assuming that 1 opaque zone and 1 translucent zone (TZ) form per year. However, the periodicity of zone formation is often un-validated and susceptible to interpretation errors and uncertainties, which propagate into age-structured calculations. The western Baltic cod Gadus morhua (WBC) stock is currently evaluated through an age-based assessment, using the typical, temperate area assumption that TZs form in autumn/winter. The periodicity of zone formation was evaluated through a mark-recapture experiment with juvenile and young adult WBC. During 2014-2017, 9111 wild cod were tagged externally (T-bar anchor tag) and internally (injection with tetracycline-hydrochloride induced a permanent fluorescent mark on their otoliths) and released. Timing of TZ formation was reconstructed through examination of otoliths from recaptured age 0-3 cod for which all required information and otoliths were available (n = 59, length range: 173-500 mm). Contrary to the assumption that TZs are formed during winter, recaptured cod consistently formed 1 TZ per year, between summer and autumn, in both juveniles (age 0-1) and young adults (age 2-3). A calendar year in the otolith is therefore composed of a TZ with opaque material on either side. Coincidence of TZ formation and recent evidence of shallow-water avoidance and reduced feeding and growth in WBC during peak summer suggests that TZs are formed when metabolic demands cannot be met. The results improve accuracy of WBC age estimates and highlight the feasibility and importance of conducting age validation experiments.

Thermoregulatory behaviour in cod: Is the thermal preference in free-ranging adult Atlantic cod affected by food abundance?

This study supports the hypothesis that well-fed cod (Gadus morhua) seek higher temperatures to increase growth rate, and poorly fed cod select lower temperatures to save metabolic energy. Depth and temperature of free-ranging adult cod (44–79 cm) were studied with data storage tags as part of a ranching project in an Icelandic fjord. Forage fish were regularly provided at four feeding stations where cod formed distinct “herds” (herd cod) that did not mingle much with the rest of the unconditioned cod in the fjord (wild cod). Several parameters (stomach fullness, liver index (fat reserves), condition factor, and growth rate) indicated that food intake was much greater in herd cod than in wild cod. In August, when the thermocline was well established, the herd cod remained in shallow (15–35 m) and warm water (8–10 °C), whereas the wild cod stayed in deep (80–90 m) and cold water (3–4 °C), but occasionally both groups explored depths and temperatures outside their preferred range. After vertical mixing in autumn when thermoregulation was not possible, the depth difference between the two groups decreased significantly.

Availability of marine collagen from Newfoundland fisheries and aquaculture waste resources

Atlantic salmon and Atlantic cod are two main finfish species harvested from Newfoundland aquaculture and wild capture fisheries, respectively. During processing, a large amount of by-products containing many valuable components are disposed of or used for low value. Lumpfish, a cleaner fish, is also not utilized at the end of the season. The proximate composition and available collagen content were investigated for Atlantic salmon and wild cod by-products and lumpfish. The highest moisture content was found in whole lumpfish (91.08%). The ash level (dry basis) was highest in fins of cod (38.14%), followed by salmon (32.19%) and lumpfish (19.06%). Salmon frames showed higher lipid content (44.81% on dry basis). The protein content (dry basis) was highest in skin of cod (96.72%), followed by salmon (71.51%) and lumpfish (77.32%). The highest collagen content was found in salmon scales and skin (51.11% and 27.45% of dry matter, respectively).

Hemodynamic Responses to Ventricular Pacing in the Fish Heart Depend on Repolarization Duration at the Paced Regions

IntroductionSpatiotemporal distribution of myocardial action potential durations is heterogeneous but a role of these heterogeneities is largely unknown. Heart ventricular contraction needs muscle mechanical heterogeneity, namely activation spread from “slow” to “fast” muscle elements, which are expected to have longer and shorter action potentials, respectively. We hypothesized that the longer repolarization duration at the primary activation area provides the better pump function. The objective of the present study was to evaluate an association between the repolarization duration in the area of initial activation and aortic blood flow under electrical pacing of different ventricular regions. Having a single ventricle and primitive, if any, conduction system, a fish heart presents a useful model for assessing the functional significance of electrophysiological nonuniformities.MethodsExperiments were performed in wild caught Atlantic cod (n = 18). Ventricular unipolar electrograms were led from an epicardial sock and intramural plunge electrodes (24–64 leads simultaneously) connected to a custom‐made mapping system. In each lead, activation times (ATs) and repolarization times (RTs) were determined as dV/dt min during QRS complex and dV/dt max during T wave, respectively. Activation‐repolarization intervals (ARIs, surrogate for action potential durations) were calculated as ARI = RT‐AT. Aortic blood flow was measured with transit time flow meter.ResultsIn sinus rhythm, the posterior wall near the atrioventricular junction was earliest to be activated (median ATs 9 [IQR 3–15] ms) and had the longest median ARIs (516 [IQR 430–580] ms). The most remote from this area regions had expectedly longer ATs (heart apex: 33 [IQR 29–44] ms; anterior base 33 [IQR 25–43] ms; both p<0.05 vs posterior wall) and also demonstrated shorter ARIs (heart apex: 482 [IQR 427–544] ms; anterior base 416 [IQR 390–497]; both p<0.05 vs posterior wall). Posterior wall pacing yielded a highest median aortic flow 47.3 [IQR 40.4–74.4] mL/min/kg. Apical and anterior basal pacing gave lower median aortic flow values (40.7 [IQR 29.0–49.2] and 29.7 [29.3–39.1] mL/min/kg, respectively; both p<0.05 vs posterior wall pacing).ConclusionPump function of the Atlantic cod heart ventricle is associated with activation‐to‐repolarization relationship: the longer repolarization at the site of activation origin provides the higher cardiac output.Support or Funding InformationThe study was supported by by the Ural Branch of the Russian Academy of Sciences (Project AAAA‐A18‐118012290365‐2) and the Program for Fundamental Research of RAS (AAAA‐A17‐117012310154‐6).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Is the Norwegian cod industry locked into a value-destructive volume logic?

Wild cod is a scarce and valuable natural resource. However, cod fishing along the coast of northern Norway has largely been about fishing as much as possible with the least possible resource effort, and thereby at the lowest cost. This traditional volume logic is rooted in biology, meteorology, and small scale capture technology. The logic is further enhanced by new large scale capture technology and a raw fish market where quality differences essentially are not reflected in the price of the fish. The intention of this paper is to address the extent to which the institutional framework in the Norwegian cod fisheries encourage or moderate this volume logic. In the paper fishing gear usage and the post-harvest industries’ product mix the last decade are analyzed. The findings show that the volume logic is still at work although this result in reduced quality of the catch landed, a product mix dominated by low-end products, and limited socio-economic value creation. This institutionalized volume logic is highly resistant to change. Moreover, the paper address how a competing and more customer-oriented quality logic can help create greater export values in Norwegian cod fisheries. Finally, implications are highlighted for how institutional measures can moderate the dominant volume logic and strengthen the emerging quality logic more in line with key policy objectives for Norwegian cod fisheries.

Field metabolic rates of teleost fishes are recorded in otolith carbonate

Field metabolic rate (FMR) is key to understanding individual and population-level responses to environmental changes, but is challenging to measure in field conditions, particularly in aquatic environments. Here we show that FMR can be estimated directly from the isotopic composition of carbon in fish otoliths (δ13Coto). We describe the relationship between δ13Coto values and oxygen consumption rate, and report results from laboratory experiments relating individual-level measurements of oxygen consumption rates to δ13Coto values in Atlantic cod (Gadus morhua). We apply our new δ13Coto metabolic proxy to existing δ13Coto data from wild cod and four deepwater fish species to test the validity of inferred FMR estimates. The δ13Coto metabolic proxy offers a new approach to study physiological ecology in free-ranging wild fishes. Otolith-based proxies for FMR are particularly promising as they allow retrospective assessment of time-integrated, individual-level FMR throughout an individual fish’s life history.

Marking otoliths of Baltic cod (Gadus morhuaLinnaeus, 1758) with tetracycline and strontium chloride

Identified were suitable dosages of tetracycline hydrochloride (TET) in three single treatments and three combined treatments of TET with 2 mg/kg strontium chloride (STR) in wild western Baltic cod (Gadus morhua), in terms of (a) obtainable mark qualities (visibility of fluorescent bands), (b) growth assessment, and (c) induced mortality rates. Isotonic NaCl solution was injected in a control group (25 cod per treatment). The results provide the basis for imperative age validation studies of Baltic Sea cod. Cod originating from pound nets near Fehmarn Island were kept in swimming net cages at the harbor of Warnemünde for 1.5 months. Mean initial total length was 28(±3) cm (salinity: 13, water temperature: 13 to 8°C). Overall average growth of surviving cod was 0.8 mm/day. In single TET treatments, lowest mortality rates and best mark quality were observed for TET concentrations of 100 compared to 50 and 25 mg/kg wet mass. Mortality rates of the 100 mg/kg treatment group were remarkably lower than in the control group emphasizing the antibiotic effect of TET. By contrast, the double treatment in the TET-STR groups resulted in a binding interaction between both markers in the fish body causing either the antibiotic potency being inhibited or TET and STR forming a non-beneficial chelate (increased mortality), and decreased incorporation of TET in the otolith (reduced visibility of TET bands). Consequently, TET (short-term marker) and STR (long-term marker) should not be injected together. Our results demonstrate that the binding interactions between these substances known from homoiotherm animals also apply for poikilotherms such as fish.

Limited evidence for differential reproductive fitness of wild Atlantic cod in areas of high and low salmon farming density

Sea cage fish aquaculture attracts large aggregations of wild fish that feed on farm waste. Fish that associate closely with farms undergo physiological changes, and captive feeding trials indicate possible negative effects on reproductive fitness. However, little is known about the significance of this phenomenon for reproduction in wild fish over larger spatial scales. To assess whether coastal areas with intensive aquaculture impact the fitness of wild fish, we collected Atlantic cod Gadus morhua from 2 areas, 1 with high and 1 with low Atlantic salmon Salmo salar farming density (HFD and LFD, respectively) in southwestern Norway, a region that hosts the world's largest coastal fish aquaculture industry. We conducted a captive spawning trial and compared a range of reproductive fitness metrics. Two fatty acids that occur at high levels in commercial feed, oleic and lineoleic acid, were strongly correlated in the ovaries of HFD fish, but a comparison of lipid profiles between HFD and LFD fish showed no differences in total lipids or essential fatty acids. Although HFD fish were slightly larger than LFD fish and had similar body condition, the volume of eggs produced did not differ, indicating relatively lower reproductive investment by HFD fish. HFD eggs were 5% smaller than LFD eggs, which did not lead to differential hatching success but may have contributed to HFD larvae being 8% smaller than their LFD counterparts at 40 d post spawning. The potential for cumulative effects of smaller eggs and larvae on fitness at later life stages warrants further investigation.