Variabilities in stomach contents of cod, collected by demersal and pelagic trawl in the southern part of the Barents Sea

Samples of Northeast Arctic cod stomachs were collected separately on demersal and pelagic stations at the same date in February 1986 and were analysed for length group 30-40cm. The results indicate that the mean value of the stomach contents weight as well as the food composition are substantially different for the two type of gear. The Spearman rank correlation coefficent (rs = 0.109;t = 0.363;P > 0.1) is not significant based on frequency data calculated from the diet of cod gathered by demersal and pelagic stations. These results question the accuracy of the consumption estimates which is based on a combination of field stomach contents weight and laboratory gastric evacuation rate studies. Therefore the mean value of the stomach contents weight should be distributed on the fishing gears, in addition to seasons, subareas, fish size and time of the day

Demonstrations of maternal effects of Atlantic cod: combining the use of unique mesocosm and novel molecular techniques - a new EU-project

One of the effects of fishing is a reduction in average age and size in exploited fish stocks, leading to an increasing proportion of recruit spawners in the stock. Current management practice assumes equal viability of offspring from first time spawners and from repeat spawners, despite the fact that that first time spawners often produce smaller eggs than older spawners. The aim of this EU-project is to follow offspring from families of first time spawning and older cod, reared under identical and semi-natural conditions in marine enclosures (mesocosms). The parental origin of the larvae is identified using microsatellite DNA methodology. The advantage of this approach, compared to traditional laboratory rearing, is that rearing conditions are close to natural conditions, and all larvae are reared in the same environment. This eliminates the tank-to-tank variability often observed in traditional rearing experiments. The fish are reared from hatching, through the larval and juvenile stages, until sexual maturity. Growth rates, survival and nutritional condition will be measured using methods such as RNA/DNA ratio and otolith micro increment analysis. The results will be related to parental origin and quality measures of the eggs. It is intended to incorporate the results into management models for improvement of fishery management strategies. In this paper we will focus on a description of the projec

Comparison of demographic and direct methods to calculate probabilistic maturation reaction norms for Flemish Cap cod (Gadus morhua)

Age and length at maturation have declined in many fish populations and this has been hypothesized to be a genetic change caused by high fishing mortality. Probabilistic Maturation Reaction Norms (PMRNs) have been used as a tool to gain a better understanding of the possible genetic nature of these changes. The demographic and direct methods are two ways to calculate PMRNs. The data requirements are more often met for the demographic method than for the direct method which requires the identification of recruit spawners. However, the demographic method relies on more assumptions than the direct method, typically assuming equality of growth and mortality rates for immature and mature individuals within an age class. This study provides the first direct comparison of demographic and direct methods and shows that both methods produce comparable results. Differences between methods are hypothesized to be owed to possible differences in growth rate between mature and immature individuals in Flemish Cap cod

Egg production of Baltic cod (Gadus morhua) in relation to variable sex ratio, maturity, and fecundity

Observed fluctuations in relative fecundity of Eastern Baltic cod (Gadus morhua L.) were related to food availability during the main feeding period and were used to develop a predictive model that explained 72% of the interannual variations in fecundity. Time series of sex ratios, maturity ogives, and relative fecundity were combined with mean weights-at-age and stock sizes from an analytical multispecies model to estimate the potential egg production (PEP). Relationships between PEP and independent estimates of realized daily and seasonal egg production from egg surveys were highly significant. The difference between estimates of potential and realized seasonal egg production was of a magnitude corresponding to the expected loss of eggs as a result of atresia, fertilization failure, and early egg mortality. The removal of interannual variability in sex ratio, maturity, and fecundity on estimates of PEP deteriorated the relationships in all three cases. PEP proved to be superior to spawning stock biomass as measure of the reproductive potential in a stock-recruitment relationship of Eastern Baltic cod. PEP in combination with the reproductive volume explained 61% of the variation in year-class strength at age 2

Assessing growth of Northeast Arctic cod by a bioenergetics model

This paper represents an attempt to elucidate how fish body size differences and metabolic costs alter the relationship between growth and consumption using a bioenergetics model. The results show that the daily growth increments of Northeast Arctic cod are considerably reduced when the consumption is adjusted for metabolism and activity cost. Consequently, the cost of the standard metabolism, specific dynamic action, swimming activity, spawning losses, egestion and excretion must be first covered before any energy is available for growth. Otherwise, individual growth rate is overestimated with extremely high food conversion efficiency

The daily feeding dynamics in various length groups of the Barents Sea capelin

A total of 692 capelin stomachs collected in the Goose Bank area in August 1989 by 24 hour fishery were analyzed by 3 cm capelin length groups. The nwnber of prey categories consumed by the Barents Sea capelin showed a tendency to increase with increasing fish length up to 12 cm. Calanus finmarchicus was the major contributor in the diet (66.4 % of total stomach weight) of length group 6-8.9 cm. Capelin in length group 9-11.9 cm had mainly fed on Calanus finmarchicus (33.5 %), Parathernisto abyssorum (14.5 %) and Thysanoessa raschii (28.0 %) while the two species Thysanoessa inermis and Thysanoessa raschii occupied 95.2% of the stomach content weight in length group 12-14.9 cm. The feeding seemed to have a peak in the evening and a minimum during the midday for all length groups. The daily ration during this period is estimated to between 1.3 and 2.2 % of fish body weight. Accordingly, these data gives between 3640 and 6160 tonnes as the daily consumption of zooplankton by the capelin stock in the Goose Bank area

An approach to estimate the daily ration of cod during intensive feeding based on 24 hour fishery

A model descnbing gashic evacuation developed by dos Santos (1990) using a modified version of the power exponential model for stomach evacuation is applied to the Northeast Arctic cod stomachs collected during 24 hour fishery. An approach to estimate the individual daily ration that takes into account the effects of initial meal size and fish weight on gastric evacuation rate in the course of 24 hour veriod is presented. The results indicate that the Northeast Arctic cod (length groups 40-44 cm and 45-49 cm) during intensive feeding on deep sea shrimp Pandalus borealis consumed between 0.50 % and 0.58 % of the body weight

Diet of polar cod, Boreogadus saida, in the Barents Sea related to fish size and geographical distribution

Polar cod stomachs collected during autumn of 1986-1988 in the Barents Sea were analysed. The results indicated that polar cod is an opportunistic feeder, and that the observed changes in feeding pattern from one region to another, is in accordance with prey availability in a particular region. The consumption increased with increasing polar cod length through an increase in prey size, rather than in the number of prey. The mean index of stomach fullness in the Northeastern part, reveal a high feeding activity in the Arctic water

The daily feeding dynamics in various lenght groups of the Barents Sea capelin during the feeding period

A total of 700 capelin stomachs collected during the feeding period in August 1989 by 24-h fishery were analyzed for each 3-cm length groups. The number of prey categories consumed by the Barents sea capelin showed a tendency to increase with increasing fish length up to 12 cm. Calanus finmarchicus was the major contributor in the diet (66.39%) of length group 6-8.9 cm. Capelin in length group 9-11.9 cm had mainly fed on Calanus finmarchicus (33.53% ), Parathemisto abyssorum (14.54%) and Thysanoessa raschii (28.04%) while the two species, Thysanoessa inermis and Thysanoessa raschii occupied 95.2% of the stomach contents weight in length group 12-14.9 cm. The evening peaks in feeding occured approximately for all length groups and the feeding was at minimum during the midday. The daily ration during the first half of August is estimated between 1.47% and 2.0% of fish body weight. Accordingly, these data gives about 4116 tonnes as the daily consumption of zooplankton by capelin stock at the Goose Bank area

Can a bioenergetics model explain growth of the northeast Arctic cod?

For the period 1984 to 1991, the individual maximum daily consumption in calories (cmax) based on 33000 cod stomach samples is given by the power equation: Cmax = 68.84 · W ^0,934 where W represents the total fish weight in grams. Daily consumption as proportion of individual maximum consumption, laboratory determination of metabolism, specific dynamic action, activity multiplier and spawning losses were incorporated into a bioenergetics model to predict growth in energy value for the Northeast Arctic Cod. Results based on 1990 data indicated that the amount of assimilated energy (90% of actual fraction of the maximum consumption) allocated for each components varied according to the season. Highest food conversion efficiency occurred during the first half of 1990 and declined during the second half. The annual rates of food turnover were within 20% - 27% of the annual consumption. Comparisons were made between four cod stocks concerning the annual energy requirements