North Sea Cod Research Articles

Intrastock differences in maturation schedules of Atlantic cod, Gadus morhua

Abstract Wright, P. J., Millar, C. P., and Gibb, F. M. 2011. Intrastock differences in maturation schedules of Atlantic cod, Gadus morhua. – ICES Journal of Marine Science, 68: 1918–1927. Differences in maturation schedules from three subpopulations of North Sea cod (Gadus morhua) were examined using the demographic probabilistic maturation reaction norm (PMRN) approach. Declines in maturation probability with size and age were evident within the North Sea cod stock, but the magnitude of decline differed among subpopulations. The difference in the rate of decline led to significant spatial differences in recent times. Changes in maturation probability could not be explained by colonization from adjacent regions indicating a local response to conditions. The greatest decline in maturation probability followed the near collapse of regional spawning biomass during the 1980s and 1990s. A new methodology was developed to integrate the effects of temperature and competitive biomass into the estimation of the PMRN. Temperature had a positive effect on maturation probability, but could only partially explain the decreasing trend in PMRN midpoints. Consequently, regional selection for early maturing genotypes provides the most parsimonious explanation for the declines in maturation probability observed. The difference in maturation probability among North Sea cod subpopulations, and the additive contribution of temperature to the estimation of change, underscores the need to account for population structuring and to incorporate temperature as a covariate in future applications of the PMRN.

Reconciling single-species TACs in the North Sea demersal fisheries using the Fcube mixed-fisheries advice framework

Abstract Ulrich, C., Reeves, S. A., Vermard, Y., Holmes, S. J., and Vanhee, W. 2011. Reconciling single-species TACs in the North Sea demersal fisheries using the Fcube mixed-fisheries advice framework. – ICES Journal of Marine Science, 68: 1535–1547. Single-species management is a cause of discarding in mixed fisheries, because individual management objectives may not be consistent with each other and the species are caught simultaneously in relatively unselective fishing operations. As such, the total allowable catch (TAC) of one species may be exhausted before the TAC of another, leading to catches of valuable fish that cannot be landed legally. This important issue is, however, usually not quantified and not accounted for in traditional management advice. A simple approach using traditional catch and effort information was developed, estimating catch potentials for distinct fleets (groups of vessels) and métiers (type of activity), and hence quantifying the risks of over- and underquota utilization for the various stocks. This method, named Fcube (Fleet and Fisheries Forecast), was applied successfully to international demersal fisheries in the North Sea and shaped into the advice framework. The substantial overquota catches of North Sea cod likely under the current fisheries regimes are quantified, and it is estimated that the single-species management targets for North Sea cod cannot be achieved unless substantial reductions in TACs of all other stocks and corresponding effort reductions are applied.

Potential bias in estimates of abundance and distribution of North Sea cod ( Gadus morhua) due to strong winds prevailing prior or during a survey

The impact of strong winds on catches of cod ( Gadus morhua) was studied using different fishing methods during small-scale surveys with commercial fishing vessels in the north-eastern central North Sea. Catch per unit effort of a flyshooter and a trawler were considerably lower in the shallower coastal water than in the deeper parts of the study area after a three week period with strong winds and rough weather conditions during the survey. At the same time, catches taken with a gillnetter showed an opposite pattern with the highest catch rates occurring at depths shallower than 50 m relative close to the coast. In another situation in which the weather conditions prior and during the survey were more moderate, the flyshooter and the trawler recorded high catch rates in the shallow coastal waters as well. Generalized Linear Model analyses revealed that wind speed prior to and during the survey had significant effects on the catch rates in particular for the trawler. These results supports fishermen's opinion that strong winds may cause an underestimation of biomass of cod in shallow waters and a bias in the resulting spatial distribution derived from bottom trawl surveys.

Infection of North Sea cod, Gadus morhua L., larvae with the parasitic nematode Hysterothylacium aduncum Rudolphi

Investigation of 2197 cod larvae and post-larvae collected in the North Sea revealed high prevalence of infection with a parasitic anisakid nematode identified morphologically and genetically as Hysterothylacium aduncum. Nematodes were third stage larvae and were almost exclusively found in the body cavity and they were never encapsulated. Prevalence increased significantly from 1992 to 2001 concomitantly with increased sea temperature. The possibility that the extent of parasitism is influenced by temperature change is discussed.

Species specific marine radiocarbon reservoir effect: a comparison of ΔR values between Patella vulgata (limpet) shell carbonate and Gadus morhua (Atlantic cod) bone collagen

A number of recent studies of the UK coastal environment have assessed the 14C marine radiocarbon reservoir effect (MRE) via quantification of ΔR values for several periods throughout the Holocene using marine mollusc shells. However, none have employed fish bone as the marine sample, and the importance of being able to use this material as a reliable dating tool is evident when considering the boom in the British fish trade during the first millennium AD, the so-called ‘fish event horizon’, and the corresponding volume of fish remains that appear in the archaeological record from this time. This study compares ΔR values derived using the multiple paired sample approach employing burnt cereal grain ( Hordeum sp.) as the terrestrial sample and either fish bone [North Sea cod ( Gadus morhua)] or marine shell [limpet ( Patella vulgata)] as the marine sample. The results show a general trend of increasing ΔR for the fish bone compared to shell, however, the differences are not statistically significant when the standard error for predicted values is used as the measure of variability in the ΔR values.

Changes in the timing of otolith zone formation in North Sea cod from otolith records: an early indicator of climate-induced temperature stress?

We examine the seasonal variation in otolith increment formation in southern North Sea cod as a means of monitoring how changes in sea temperature over the past 20 years have affected cod in the wild. Seasonal opaque zone formation was related to winter and early spring. Timing of opaque zone formation was not influenced by either temperature or fish length, but increasing age led to slightly earlier but slower opaque zone formation. In contrast, there was a clear shift in the timing of translucent zone formation with temperature. In warm years, translucent growth occurs up to 22 days earlier than in colder periods. Increasing age and smaller size-at-age resulted in an earlier transition from opaque to translucent edge formation. Translucent zone formation appears indicative of increasing metabolic stress, and the earlier onset provides direct evidence of the impact of increasing sea temperatures on wild North Sea cod stocks.

Spawning stock and recruitment in North Sea cod shaped by food and climate

In order to provide better fisheries management and conservation decisions, there is a need to discern the underlying relationship between the spawning stock and recruitment of marine fishes, a relationship which is influenced by the environmental conditions. Here, we demonstrate how the environmental conditions (temperature and the food availability for fish larvae) influence the stock–recruitment relationship and indeed what kind of stock–recruitment relationship we might see under different environmental conditions. Using unique zooplankton data from the Continuous Plankton Recorder, we find that food availability (i.e. zooplankton) in essence determines which model applies for the once large North Sea cod (Gadus morhua) stock. Further, we show that recruitment is strengthened during cold years and weakened during warm years. Our combined model explained 45 per cent of the total variance in cod recruitment, while the traditional Ricker and Beverton–Holt models only explained about 10 per cent. Specifically, our approach predicts that a full recovery of the North Sea cod stock might not be expected until the environment becomes more favourable.

The importance of predator–prey overlap: predicting North Sea cod recovery with a multispecies assessment model

Abstract Kempf, A., Dingsør, G. E., Huse, G., Vinther, M., Floeter, J., and Temming, A. 2010. The importance of predator–prey overlap: predicting North Sea cod recovery with a multispecies assessment model. – ICES Journal of Marine Science, 67: 1989–1997. The overlap between predator and prey is known as a sensitive parameter in multispecies assessment models for fish, and its parameterization is notoriously difficult. Overlap indices were derived from trawl surveys and used to parametrize the North Sea stochastic multispecies model. The effect of time-invariant and year- and quarter-specific overlap estimates on the historical (1991–2007) and predicted trophic interactions, as well as the development of predator and prey stocks, was investigated. The focus was set on a general comparison between single-species and multispecies forecasts and the sensitivity of the predicted development of North Sea cod for the two types of overlap implementation. The spatial–temporal overlap between cod and its predators increased with increasing temperature, indicating that foodweb processes might reduce the recovery potential of cod during warm periods. Multispecies scenarios were highly influenced by assumptions on future spatial overlap, but they predicted a considerably lower recovery potential than single-species predictions did. In addition, a recovery of North Sea cod had strong negative effects on its prey stocks. The consequences of these findings for management are discussed.

Bromine patterns in Norwegian coastal Cod otoliths—a possible marker for distinguishing stocks?

Bromine was found to accumulate in otoliths of Norwegian coastal Cod Gadus morhua that were reared under known conditions. Despite the fact that the Cod were moved from one rearing environment to another, causing marked changes in some otolith elemental concentrations, bromine appeared to accumulate continuously along certain growth axes as revealed by 2-D elemental mapping. In contrast, North Sea and Baltic Sea Cod showed little to no patterning in Br. We suggest that Br uptake in otoliths may be under physiological and genetic control, and as such, may prove useful as a stock identification tool.

Performance of harvest control rules in a variable environment

AbstractBrunel, T., Piet, G. J., van Hal, R., and Röckmann, C. 2010. Performance of harvest control rules in a variable environment. – ICES Journal of Marine Science, 67: 1051–1062. Population dynamic models used for fisheries management assume that stocks are isolated entities, ignoring the influence of environmental factors on stock productivity. An operating model parameterized for North Sea cod, plaice, and herring is developed, in which the link between recruitment and environment is assumed to be known and described by generalized additive models. This tool is used to compare the performance of harvest control rules (HCRs) when recruitment is independent of the environment or when recruitment is affected by an environment varying according to different scenarios. The first HCR exploited the stock with a fixed fishing mortality (F) corresponding to maximum sustainable yield, and in the second HCR, F was set equal to the precautionary approach F (i.e. Fpa), but reduced from Fpa when stock biomass fell below Bpa. The performance of the HCRs altered only slightly in a randomly varying environment compared with a constant one. For a detrimental change in the environment, however, no HCR could prevent a massive decrease in stock size. The performance of the HCRs was also influenced by the stock characteristics, such as recruitment variability or the shape of the stock–recruitment relationship. The performance of “environmental” HCRs (eHCRs), in which F varies depending on environmental conditions, was compared with that of conventional HCRs. The gain in using the eHCR was small, except for a detrimental change in the environment, where the eHCR performed markedly better than a conventional HCR. The benefits of using the eHCR were the greatest for the stock with the strongest environment–recruitment relationship.

Spatial changes in the sensitivity of Atlantic cod to climate-driven effects in the plankton

Recent strategies to sustain fish stocks have suggested a move towards an ecosystem based fisheries management (EBFM) approach. While EBFM considers the effect of fishing at the ecosystem level, it generally struggles with climate-driven environmental variability. In this study we show that the position of a fish stock within its distributional range or thermal niche (we use Icelandic and North Sea cod as examples of stocks at the centre and edge of their niche, respectively) will influence the relative importance of fishing and climate on abundance. At the warmer edge of the thermal niche of cod in the North Sea, we show a prominent influence of climate on the cod stock that is mediated through temperature effects on the plankton. In contrast, the influence of climate through its effects on plankton appears much less important at the present centre of the niche around Iceland. Recognising the potentially strong effect of climate on fish stocks, at a time of rapid global climate change, is probably an important prerequisite towards the synthesis of a cod management strategy.

Evaluating the performance of survey-based operational management procedures

The design and evaluation of survey-based management strategies is addressed in this article, using three case-study fisheries: North Sea herring, Bay of Biscay anchovy and North Sea cod, with a brief history and the main management issues with each fishery outlined. A range of operational management procedures for the case study stocks were designed and evaluated using trends that may be derived from survey indices (spawner biomass, year-class strength and total mortality) with an array of simple and more structured observation error regimes simulated. Model-free and model-based indicators of stock status were employed in the management procedures. On the basis of stochastic stock-specific simulations, we identified the following key determinants of successful management procedures: (i) adequate specification of the stock-recruit relationship (model structure, parameter estimates and variability), (ii) knowledge of the magnitude and structure of the variation in the survey indices, and (iii) explication of the particular management objectives, when assessing management performance. More conservative harvesting strategies are required to meet specified targets in the presence of increasing stochasticity, due to both process and observation error. It was seen that survey-based operational management procedures can perform well in the absence of commercial data, and can also inform aspects of survey design with respect to acceptable levels of error or bias in the surveys.

Recruitment of North Sea cod (Gadus morhua) and Norway pout (Trisopterus esmarkii) between 1992 and 2006: the interplay between climate influence and predation

The interplay between temperature-related processes and predation in determining age-1 recruitment strength between 1992 and 2006 was analysed for North Sea cod ( Gadus morhua ) and Norway pout ( Trisopterus esmarkii ). For this purpose, an predation impact index (PI) was calculated out of survey data. PI was assumed to depend on the abundance of the predators and on the spatial overlap between predator and prey populations. Generalized additive models (GAMs) were created with spawning stock biomass (SSB) and sea surface temperature (SST) in the respective spawning and nursery areas and PI as explaining variables. SSB had no significant impact on recruitment during this time period for both species. SSTs during spring and PI explained the interannual variability in recruitment strength to a large extent (88% of the total variance for cod and 68% for Norway pout). The SST during spring determined the overall level of recruitment. At SSTs above a certain level, however, the effect on recruitment was no longer significant. In these temperature ranges, predation was the dominant effect. Therefore, the fate of North Sea cod and Norway pout stocks under global-warming conditions will be strongly influenced by the status of the North Sea food web.

A generic model to estimate food consumption: linking von Bertalanffy’s growth model with Beverton and Holt’s and Ivlev’s concepts of net conversion efficiency

In this paper, a mathematical derivation is presented that links von Bertalanffy’s growth model with the concept of net conversion efficiency of Beverton and Holt, aiming at the development of an equation that can calculate food consumption rates of wild populations from parameters of the von Bertalanffy growth equation and an estimate of the net food conversion efficiency of Beverton and Holt. The derivation is based on Pauly’s version of the generalized von Bertalanffy equation, which allows the allometric exponent of the anabolism term to differ from 2/3, as in the standard von Bertalanffy equation. As a side product, a general model is formulated that describes the gross growth conversion efficiency (K1 of Ivlev) as a function of weight of the organism. The new equations for the estimation of food consumption are applied in two case studies, North Sea cod ( Gadus morhua ) and whiting ( Merlangius merlangus ), for which a variety of consumption estimates is available from conventional gastric evacuation-based methods. The new method produces results that show a similar degree of variability as was observed in various applications of the gastric evacuation method.

Using min/max autocorrelation factors of survey-based indicators to follow the evolution of fish stocks in time

Fisheries research monitoring surveys provide an ensemble of measurements on fish stocks and their environment. Because the interannual variability in such survey-based indicators is high and because diagnostics on fish stocks cannot be based on noise, our concern is to make use of what is continuous in time to obtain a reliable diagnostic. In this paper, we show how min/max autocorrelation factors (MAFs) can be useful for assessing the status of a fish stock. Indeed, MAFs will allow us to (i) summarize the multivariate indicator signals into orthogonal factors that are continuous in time, (ii) select those indicators that carry the major signal in time, and (iii) forecast stock status by modelling the time continuity of the MAFs. These different potential uses of MAFs in an indicator-based approach to assessment were illustrated with North Sea cod, for which a suite of biological and spatial indicators are available over a 21-year survey series.

Notes on nine biological indicators estimable from trawl surveys with an illustrative assessment for North Sea cod

This paper reviews a selection of nine indicators for assessing the biological state of a fish population monitored only by using trawl surveys in which catchability is assumed constant from time to time and place to place. The intention is to inform or remind of the range of possibilities and of associated practical and statistical issues. The indicators, grouped according to relationships with abundance, length, age, weight, and reproduction, are documented in a standard format covering estimation, interpretability, sensitivity, considerations for reference points where possible, and an example of their application. Lastly, selected indicators together with year-class curves fitted to abundance indices-at-age are applied to the North Sea cod to demonstrate the feasibility of an assessment based on only one trawl survey. Results support findings of the International Council for the Exploration of the Sea (ICES) working group on the assessment of demersal stocks in the North Sea and Skagerrak.

Thermal acclimation to 4 or 10°C imparts minimal benefit on swimming performance in Atlantic cod (Gadus morhua L.)

Thermal acclimation is frequently cited as a means by which ectothermic animals improve their Darwinian fitness, i.e. the beneficial acclimation hypothesis. As the critical swimming speed (U (crit)) test is often used as a proxy measure of fitness, we acclimated Atlantic cod (Gadus morhua) to 4 and 10 degrees C and then assessed their U (crit) swimming performance at their respective acclimation temperatures and during acute temperature reversal. Because phenotypic differences exist between different populations of cod, we undertook these experiments in two different populations, North Sea cod and North East Arctic cod. Acclimation to 4 or 10 degrees C had a minimal effect on swimming performance or U (crit), however test temperature did, with all groups having a 10-17% higher U (crit) at 10 degrees C. The swimming efficiency was significantly lower in all groups at 4 degrees C arguably due to the compression of the muscle fibre recruitment order. This also led to a reduction in the duration of "kick and glide" swimming at 4 degrees C. No significant differences were seen between the two populations in any of the measured parameters, due possibly to the extended acclimation period. Our data indicate that acclimation imparts little benefit on U (crit) swimming test in Atlantic cod. Further efforts need to identify the functional consequences of the long-term thermal acclimation process.

Effect of bottom type on catch rates of North Sea cod ( Gadus morhua) in surveys with commercial fishing vessels

Seven surveys with commercial fishing vessels were conducted during a collaborative fishermen–scientist project on the distribution of cod in the north-eastern North Sea between June 2006 and June 2008. A flyshooter, a trawler and a gillnetter participated in this study. In general, catch rates were substantially higher on gravel or stone bottom and at ship wrecks than on sand bottom. The difference in the catch rates between the two bottom categories at paired stations within a short distance was highly significant for all the three fishing methods. Similarly, average CPUE for most surveys was several times higher on rough than on smooth bottom. These differences were highly significant for early autumn surveys conducted with the flyshooter and trawler and all gillnet surveys, the summer surveys for the flyshooter and the gillnetter, but not for the winter surveys with the trawler and the flyshooter. The latter suggest that bottom type preference may change with season, e.g. with respect to spawning migrations in winter and in relation with changes in the availability of food during spring and summer.

Book Review: An Environmental History of North Sea Ling and Cod Fisheries 1840–1914

Book Review: An Environmental History of North Sea Ling and Cod Fisheries 1840–1914

Geolocation of North Sea cod (Gadus morhua) using hidden Markov models and behavioural switching

When geolocating fish based on archival tag data, a realistic assessment of uncertainty is essential. Here, we describe an application of a novel Fokker–Planck-based method to geolocate Atlantic cod ( Gadus morhua ) in the North Sea area. In this study, the geolocation relies mainly on matching tidal patterns in depth measurements when a fish spends a prolonged period of time at the seabed with a tidal database. Each day, the method provides a nonparametric probability distribution of the position of a tagged fish and therefore avoids enforcing a particular distribution, such as a Gaussian distribution. In addition to the tidal component of the geolocation, the model incoporates two behavioural states, either high or low activity, estimated directly from the depth data, that affect the diffusivity parameter of the model and improves the precision and realism of the geolocation significantly. The new method provides access to the probability distribution of the position of the fish that in turn provides a range of useful descriptive statistics, such as the path of the most probable movement. We compare the method with existing alternatives and discuss its potential in making population inference from archival tag data.