Fluctuation in stock properties of north-east Arctic cod related to long-term environmental changes

From a historic perspective, the north‐east Arctic cod stock, which is found in the Barents Sea–Svalbard region, has been the most productive gadoid stock in the Atlantic. Variation in catch has always been large, but during the last 10–15 years catch and stock abundance have reached the lowest level on record. Three major causes of variation have been discussed: (i) stock reduction through exploitation; (ii) environmental influences on recruitment; and (iii) species interaction effects on maturation, growth and mortality. In addition, interactions among these three sources might be important. The influence of each specific factor is difficult to evaluate from incidental observations and short‐term time series. In this respect, the time series on catches and on biological and environmental information of this stock, which partly extend back to the 19th century, occupy a unique position in comparison to data on most other stocks. In this paper, fluctuations in catch and stock abundance are compared with changes in recruitment, size/age and growth. This information is discussed in view of historic variation in ecology and environment. The stock has been under particularly high exploitation pressure since the mid‐1970s. Further, large changes in growth rates and poor recruitment to the commercially exploited stock are characteristic of late 1980s and throughout the 1990s. The analysis shows that substantial long‐term variation might underlie short‐term variability, and more importantly, that long‐term changes roughly coincide with similar fluctuations in the environment. Such factors might substantially affect the relationship between spawning stock and recruitment, which is also apparent from the difference in conclusions reached by various published studies. Consequently, it is suggested that using a steady‐state perspective for the population dynamics may lead to mismanagement and to a reduction of the long‐term yield from this stock.     

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

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

Match/mismatch predictions of spawning duration versus recruitment variability

According to the match/mismatch hypothesis, larval fish survival and eventual recruitment is dependent on the offset time between the peaks of abundance of larvae and their planktonic prey. A rudimentary larval food supply model is developed to determine the dependence of food availability on the mismatch between peaks. The model predicts that recruitment variability should increase as spawning duration decreases, a result which is moderately supported by an analysis of Atlantic cod (Gadus morhua) data.     

Stability of water masses-impact on cod recruitment off West Greenland

Based upon the hypothesis of Meyer (1968: Hansa, 16:2–4) that periods of low stability of the water column in autumn should parallel good cod year-classes in the following year, the paper analyzes available time series of cod recruitment, subsurface oceanographic stability data and wind stress data off West Greenland. No significant correlations were found between stability, wind stress and recruitment when considering the entire. length of available time series. However, splitting the autumn-based oceanographic data set into a pre- 1970s part (when warm climatic conditions and high recruitment prevailed), and a recent part (for the past 20 years of cooling climate) yields significant coherence of recruitment and environmental conditions for the cold climate period. For the warm climate period, only occasional consistency is shown. The possible influence of the Great Salinity Anomaly on recruitment of cod is discussed. It is suggested that during the past 20 years a new correlation mechanism, an advective coupling instead of low stability coupling, may explain the variability of recruitment.     

Effects of dietary protein and lipid level, and water temperature, on the post-feeding oxygen consumption of Atlantic cod and haddock

Tank respirometry was used to study the interactive effects of protein:lipid level (55%:11% vs. 42%:16%; both diets isoenergetic) and temperature (11, 6 and 2 °C) on the magnitude and duration of specific dynamic action (SDA) in juvenile Atlantic cod and haddock. The protein:lipid level did not affect any measured variable. However, numerous temperature and species effects were observed. For example, although the maximum post-feeding oxygen consumption (30–50% above routine metabolic rate; RMR) and SDA duration (∼55–85 h; SDA_DUR) were not affected by temperature, SDA_DUR g⁻¹ of food increased from 11 to 2 °C (from ∼3 to 12 h g food⁻¹). While absolute SDA (mg O₂) decreased by ∼60–65% in cod and ∼75% in haddock from 11 to 2 °C, due to a concomitant decrease in food consumption from ∼2.0% to 0.6% body mass, SDA comprised between 3.3% and 5.2% of the dietary energy content at all temperatures. Finally, RMR at 11 and 2 °C and SDA_DUR at 2 °C were 25–35% and 25% greater in cod, respectively, as compared with haddock. These results suggest that feeding reduced protein diets at low water temperatures is unlikely to improve the growth of these species.     

Weight-based yield per recruitment and spawning-biomass per recruitment analyses of pacific cod <Emphasis Type="Italic">Gadus macrocephalus</Emphasis> off the Pacific coast of southern Hokkaido,Japan

ABSTRACT:   The present study assessed the stock state of Pacific cod Gadus macrocephalus caught off the coast of southern Hokkaido, Japan. Weight-based yield per recruitment (YPR) and spawning-biomass per recruitment (SPR) analyses were used for this assessment. The current fishing mortality (average from 1998 to 2000) was 0.65 and weight at first capture was 0.5 kg bodyweight. Under these fishing pressures, the YPR of Pacific cod in southern Hokkaido was 1.06 kg/recruitment and percentage of SPR (%SPR) was 6.9%. The %SPR was lower than the critical limit at 20%SPR. The main reason that values of both YPR and %SPR were not optimum, would be that the weight at first capture was too small. Raising the weight at first capture was thought to be a better strategy from the biological viewpoint, and reducing fishing mortality to 0.3 would be the next alternative strategy from the fisheries management viewpoint.     

Relationships between temperature and landing patterns in the southern Labrador and eastern Newfoundland inshore cod fishery

The temporal pattern of cod landings in the inshore trap fishery off the east coast of Newfoundland and Labrador is examined in this study. Unlike previous studies which have been broad scale or of a localized nature, this analysis looks at variations within fishing seasons and among fishing areas in trap landings. A significant relationship between water temperature and the weekly pattern is demonstrated. Significant spatial patterns in weekly landings are described and related to longshore differences in the seasonal variation in temperature.     

The effects of stress and storage temperature on the colour and texture of pre-rigor filleted farmed cod (Gadus morhua L.)

This experiment deals with the effects of pre‐slaughter stress and storage temperature on muscle pH, fillet contraction, colour and texture in pre‐rigor filleted farmed cod fillets. The fish were either sedated with a low dose of MS‐222 (14.3 mg L^?1) (unstressed groups) or exposed to the air for 3 min (stressed groups) before being submerged in a benzocaine bath (150 g L^?1). The fish were then killed by a blow to the head, their gills cut, filleted and finally stored at either 4 or 20°C. The stressed groups had significantly lower pH values after slaughter (pH=7.0) than the unstressed groups (pH=7.3). This difference was maintained until post rigor for the fish stored at 4°C, but at 20°C it was immediately overshadowed by a decrease in pH caused by temperature‐dependent processes. The length contraction and changes in registered colour values were more pronounced at both the higher temperature and the higher level of pre‐slaughter stress. Again temperature dominated, but significant and consistent effects were registered from stress. No significant effects of stress on texture post rigor were observed. It is concluded that high storage temperature masks the majority of effects caused by pre‐slaughter stress on the measured variables. Stress management protocols, however, are important when the fillets are kept at the common storage temperature of 4°C.     

Growth and survival of Arcto-Norwegian and Norwegian coastal cod larvae (Gadus morhua L.) reared together in mesocosms under different light regimes

To evaluate genetic differences in growth and survival potential as a function of light regime, cod larvae from two stocks (CC: coastal cod and AC: Arcto-Norwegian cod, spawning at latitudes of 60°N and 69°N respectively) were co-reared in mesocosms. The experiment was carried out with two artificially illuminated light regimes, corresponding to the natural photoperiod at peak spawning of the respective stocks (CC: 7–8 h distinct dark period, AC: ‘mid-night sun’ as continuous light with 2–3 h reduced light intensity). One additional regime with ambient natural light analogous to 60°N was used. At termination on day 36 post-hatch, larvae of the co-reared stocks were distinguished from each other by a genetic marker in the CC stock. Stock-specific differences in growth rate and survival were indicated, with CC larvae growing and surviving better than AC larvae. This is in agreement with earlier comparisons of larval growth in these stocks. The 69°N light regime did not enhance growth for any of the two stocks. However, natural light greatly increased growth rate of both stocks, indicat ing important mechanisms in regulation of larval feeding relative to light quality or intensity. The results may be important for choice of stock and illumination in intensive aquaculture of this species.     

Seasonal shift in spawning of Atlantic cod (Gadus morhua L.) by photoperiod manipulation: egg quality in relation to temperature and intensive larval rearing

Commercial intensive fry production of Atlantic cod will be dependent on production of viable eggs independent of season. This can only be done by manipulation of maturation by photoperiod, but little is known about potential effects on egg characteristics and larval viability. In two cod broodstocks, maturation was successfully advanced or delayed 6 months compared with normal spawning season (March–April) by manipulation of photoperiod. The advanced broodstock spawned both in spring and autumn the same year. In two of the spawning tanks during autumn, ambient temperature was reduced after reaching 13.7°C during the first half of the spawning period. Egg quality and viability were monitored, and several egg batches were incubated, hatched and start‐fed for examination of growth and survival. Temperatures above 9.6°C resulted in significant reductions in fertilization and normal egg development. Concurrently, fractions of dead and unfertilized eggs increased with elevated temperature. Actual relative fecundity was not affected by temperature. Egg characteristics improved when temperature was controlled and lowered below 9.6°C. Occurrence of irregular spawners suggests that handling of broodstock fish should be avoided during maturation and spawning. Cod larvae originating from eggs of the advanced or delayed broodstocks were successfully reared beyond metamorphosis. Survival was 9.0–46.6% and 29.3% in green and clear water respectively. Survival correlated with both initial and average feeding conditions, but growth rate did not correlate with either of survival and feeding conditions. Specific growth rates (8.3–13.6% day⁻¹) is comparable with other intensive rearing trials with cod, but were lower than reported from nature‐like systems.     

Efficacy of egg surface disinfectants in captive spawning Atlantic cod Gadus morhua L. and haddock Melanogrammus aeglefinus L.

In an effort to optimize the efficiency of high‐density incubation of Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus eggs, the per cent hatch of eggs treated with four disinfectants (3% hydrogen peroxide, 1% polyvinylpyrrolidone iodine, 0.1% sodium hypochlorite and a 0.005% antibiotic solution – penicillin/streptomycin) was compared in both species. The per cent hatch of eggs of each species was greatest after a 24 h treatment with the antibiotic solution. The hatching success of eggs treated within the different disinfectant treatments depended upon the embryonic developmental stage in both species. The sodium hypochlorite treatment had the lowest % coverage of colony growth after disinfected haddock eggs were plated onto sterile agar media, highest survival rates to the end of the embryonic period, but the lowest per cent hatch.     

Investigation of the temporal effects of spawning season and maternal and paternal differences on egg quality in Atlantic cod Gadus morhua L. broodstock

A better understanding of the parameters affecting egg quality and larval survival is of importance for continued development of cod broodstock and efficient husbandry practices. Decision tree analysis (DTA) was applied to analyse 3 years of egg quality data in an effort to extract the most important variables (i.e. predictors) in explaining differences in egg quality. The effect of three predictors (spawning time, maternal and paternal differences) has been studied on early cleavage pattern parameters, egg diameters, fertilization and hatching rates and has shown that females are the dominant variable and that time has a limited and inconsistent impact on the data. When using maternal, paternal differences and batch number (instead of spawning time) as predictors, the results confirm that no particular relationship is found between batch order (i.e. order in time) and egg quality. Moreover, batches with a higher egg quality show a consistency in the parameters assessed (i.e. batches with higher rates of normality in any parameter tend to be normal for other parameters). This is confirmed by the significant correlations found between cleavage parameters. Our results highlight that spawning time is of less importance than female parent contribution in ensuring high rates of fertilization and larval hatch, and maximizing general egg quality.     

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

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

The distribution of cod (Qadus morhua) in relation to temperature and oxygen level in the Gulf of St. Lawrence

Bottom temperature and oxygen level were measured in the northern Gulf of St. Lawrence during a ground-fish trawl survey held in August and September 1991. Cumulative relative frequencies of temperature and oxygen level and cumulative relative frequencies of temperature x cod and oxygen X cod were computed with consideration of the random stratified sampling scheme. A total of 84-3% of temperature x cod frequency cumulated from 2^oC to 4^oC, but only 25% of the frequency of temperature alone cumulated from 2^oC to 4^oC. At an oxygen level of 3.4 mg/1, only 2.6% of the oxygen X cod frequency had cumulated, while 24.3% of the cumulative frequency of oxygen alone was already observed at the same oxygen level. A comparison of the cumulative relative frequencies of temperature and of temperature X cod indicates that cod choose intermediate temperatures in the available range. A comparison of the cumulative relative frequencies of oxygen and of oxygen x cod indicate that cod avoid water with an oxygen level below 3.4 mg/1. This oxygen level is known to cause physiological stress in captivity. Data from the stations with oxygen levels ± 3.4 mg/1 were ignored, and the cumulative relative frequency of temperature was recomputed. As a result, the avoidance of cold water by cod was emphasized: 23% of the cumulative frequency of temperature alone was below 2^oC, and 47.8% was above 4^oC, in contrast to 17.2% and 57.5%, respectively, prior to removal of data from hypoxic stations.     

Environmental influences on melatonin secretion in Atlantic cod (Gadus morhua L.) and their relevance to commercial culture

This study investigated the effects of photoperiod and temperature on plasma melatonin secretion in Atlantic cod (Gadus morhua L.). Initial work confirmed the presence of a diel profile of melatonin synthesis, with elevated levels during the dark phase. Unusually for fish, the peak in plasma melatonin occurred towards the end of the dark phase, which is indicative of a type `A' melatonin profile. When exposed to 60 hours of continuous darkness a clear endogenous rhythm of melatonin synthesis was observed, which continued for 4 cycles with a periodicity which, approximated to 24 h. When acclimated to varying temperatures (4, 8, 12 or 16 °C) no variation in melatonin production was seen, however, body size appeared to be an important influence, with the smallest fish exhibiting significantly higher levels of dark phase melatonin. Finally, the application of additional night-time illumination to cod maintained in sea cages i.e. without blackout, did not significantly reduce dark phase plasma melatonin, suggesting that cod are less sensitive to photoperiod manipulation in cages than salmonids.     

Effect of acclimation temperature on the acute stress response in juvenile Atlantic cod, Gadus morhua L., and haddock, Melanogrammus aeglefinus L.

Juvenile Atlantic cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L.) were subjected to 30 s air exposure stressors following acclimation to 4, 10 and 14 and 4, 8 and 14°C respectively. Both species responded to the stressor with increases in plasma cortisol at all temperatures tested. At 14°C cortisol levels peaked within 1 h post‐stressor, and returned to pre‐stressor levels within 24 h. In contrast, at 4°C, peak cortisol levels were not attained until 6 h post‐stressor in haddock and remained elevated beyond 24 h in both species. The rate of plasma glucose accumulation was greater at higher temperatures in both species and no increase was seen at 4°C. Lysozyme activity in cod, in response to the stressor, was lower than the values reported for some other species and increased slightly at 14°C. The results show that cod and haddock acclimated to different temperatures respond to common, acute stressors in a manner similar to other teleosts.     

Variation in fish condition between Atlantic cod (Gadus morhua) stocks, the effect on their productivity and management implications

A comparative analysis of the fish condition (Fulton’s K) of 10 cod stocks in the north Atlantic in relation to the temperature of their habitat, growth rates and their reproductive potential is presented. It is shown that the cod stocks in the north Atlantic display different levels of mean condition, which is partly due to the different temperature regimes of their habitats. Cod living in colder waters, e.g. Greenland, Labrador and Grand Bank stocks, were found to be in poorer condition than cod living in warmer waters, e.g. North Sea and Irish Sea stocks.

Poor condition causes reduced productivity in terms of slow growth and low recruitment potential. Stocks in better condition display significantly higher weights at age 4 than stocks in poor condition. The a coefficients (function’s slopes) obtained from standardised Ricker’s recruitment–spawning stock biomass (SSB) relationships were defined as indicators for the recruitment potential of stocks. These a coefficients were found to be positively correlated with the mean condition factor of the 10 stocks analysed. This indicates that stocks consisting of individuals in poor condition appear to be very susceptible to reduced recruitment at low SSB, while the stocks that consist of fish in good condition seem to behave more robustly with a higher probability of good recruitment at low SSB. The positive effect of the cod condition on their reproductive potential generally implies that the stocks in good condition in the temperate regions of the northeast and west Atlantic can sustain higher exploitation rates than stocks in poor condition in the colder regimes of the northwest Atlantic (Greenland, Labrador and Grand Bank). This is confirmed by the positive relationship established between the estimated biological management reference points F_med and the mean cod condition factors, as well as by the recent status of these stocks.     

Effects of dietary levels and ratio of phosphatidylcholine and phosphatidylinositol on the growth,survival and deformity levels of Atlantic cod larvae and early juveniles

Effects of two weaning diets that differed in phospholipid (PL) classes on growth, survival and deformities of cod larvae and early juveniles were evaluated. Cod larvae were fed rotifers until 21 days post hatch (dph) and then weaning onto dry diet started. One group of larvae were fed a control diet with low levels of phosphatidylcholine (PC), PE and phosphatidylinositol (PI) and the other group of larvae were fed with an experimental diet containing higher levels of PC, PE and PI. Larvae fed with the control diet were significantly smaller than larvae fed with the experimental diet at the end of the experiment. Swim bladder abnormalities were significantly higher in larvae fed with control diet at 35 dph than the larvae fed with experimental diet; however, no significant difference was evident at 42 dph. Vertebral deformities were significantly higher in larvae fed with control diet and scoliosis was significantly different between the treatments. Survival was also significantly higher in the experimental group. Our results indicate that dietary levels of PL, PC and PI may affect the cod larval growth, survival and deformities. More detail studies are needed to find out the optimal levels of these important PL classes in larval cod diets.     

Length-based Beverton and Holt spawning stock biomass per-recruit-models, with application to the Lates stappersii (Boulenger) stock in Lake Tanganyika

Two kinds of length-based Beverton and Holt spawning stock biomass per-recruit ( SSB/R )-models are presented. The first allows the computation of the absolute value of SSB/R . The second estimates the index of SSB/R , also called 'relative SSB/R '. Both models are extensions of the original age-based Beverton and Holt SSB/R -models. The SSB/R and its index can be assessed with reference to variations in the exploitation characteristics. These models were applied to the Lates stappersii (Boulenger) stock in Lake Tanganyika using fishing mortality as the independent variable. They produce an absolute value of SSB/R of 12.95 g, representing 22.4% of that of the 'virgin' stock for the 1993–1994 equilibrium-fishing regime. This level is indicative of an unsafe state of the stock in question. Moreover, the SSB for L. stappersii seems to be fragile under increased fishing pressure as, for example, doubling or tripling the current fishing regime would reduce the current SSB/R by 57.3 and 77.4%, respectively. In general, the use of length-based Beverton and Holt SSB/R -models is handicapped by underlying hypotheses and sub-models which are restrictive.     

Discrimination between Icelandic cod (Gadus morhua L.) populations from adjacent spawning areas based on otolith growth and shape

Studies on Atlantic cod (Gadus morhua L.) have demonstrated extensive variation in life history characteristics among populations exposed to different environmental conditions. In Iceland, cod sampled from adjacent spawning areas, within the main spawning area at the southwest coast, exhibit variation in vital life history characteristics. In this study, we investigated the stock structure of Icelandic cod on the main spawning grounds to discriminate between these adjacent spawning groups by using otolith growth and shape. Otolith growth was based on annual increment width estimated for the major age groups (6–7 year olds) of the spawning stock. Otolith shape was determined using Fourier analysis and compared among cod sampled from the different spawning areas with stepwise canonical discriminant analysis (CDA). We found significant differences in growth and otolith shape between adjacent spawning groups of cod where those sampled close to the coast differed from those that were sampled further out on the bank and continental shelf. Our results suggest that these cod may belong to different populations and the large and fast growing cod spawning in the coastal area may need special protection given their significance to the overall productivity of the stock.