In this study, we investigated whether otolith weight can be used to estimate fish age with the same level of accuracy as that of the traditional annuli counting technique in a commercially important species such as haddock (Melanogrammus aeglefinus). Results indicate that this method is highly effective for young fish (around 97% correct classification), whereas its powers of prediction decrease with the increasing age of the fish. For older fish, the otolith weight cannot be an accurate estimator of fish age if the weight overlap between the different age classes is too large. Nevertheless, the otolith weight technique is strictly dependent on correct age determination through the counting of annuli of those individuals used in the calibration. Hence, an increase in the accuracy of ageing obviously determines an increase of the power of otolith weight to estimate fish age. Therefore, we suggest that otolith weight could represent a routine technique for determination of the age structure of haddock populations. This technique has the merit to be objective, fast, 100% repeatable and has the same level of accuracy as that of annuli counting.

No field studies have been performed on the selectivity of herring and sprat in the southern Baltic Sea in relation to their entire range of prey. Accordingly, we tested in the field the following hypotheses: (i) sprat and herring are selective feeders and (ii) sprat and herring selectivity is size- and season-dependent. The results show that (i) smaller herring and all size classes of sprat are strictly zooplanktivorous. selecting principally Temora longicornis and Bosmina maritima during the autumn and Pseudocalanus elongatus in winter; (ii) larger herring are essentially nektobenthos feeders, predating on Mysis mixta during the autumn and amphipods and polychaetes during the winter; and (iii) herring and sprat seem to avoid Acartia spp. in both autumn and winter. During the autumn, herring are zooplanktivorous up to 18-20 cm, whereas in winter herring feed on nektobenthos starting from 14-15 cm. Selectivity was not an absolute process but it was related to prey relative abundance in the sea and, possibly, to prey profitability (e.g. size, conspicuousness. and reaction time). (C) 2004 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.

Hypolimnetic oxygen injection is a management tool used to improve water quality by preventing anoxia and associated phosphorus release from the sediments. An additional benefit would be formation of a low-oxygen refuge for large-bodied herbivorous zooplankton in the metalimnion. The magnitude and timing of hypolimnetic oxygenation was deliberately manipulated during summer 1997 in Irondequoit Bay, a eutrophic Lake Ontario embayment (New York, USA), to maintain metalimnetic dissolved oxygen (DO) concentrations in the range of 1–2 mg/L. We assessed the vertical distributions of fish and zooplankton in 1997, a year with controlled oxygenation, and 1996, a year with oxygenation but without a deliberate attempt to create amenable refuge conditions. After initiation of oxygenation, June through early August metalimnetic DO in 1997 was 1.8 ± 0.5 mg/L (means ± 2 se), whereas metalimnetic DO in 1996 was 3.8 ± 1.0 mg/L. Hydroacoustic surveys in 1997 and gillnet catches in both years indicated that >96% of planktiv...

We analysed Baltic Sea pelagic fish (herring and sprat) spatial and temporal distribution, size distribution at different depths and time of the day and diel feeding pattern. In 1995 the study area was investigated by acoustic survey for 3 d, 3, 4 and 11 October, to investigate spatial and temporal distribution of pelagic fish. The area was divided in four different transects forming a survey quadrate of 15 nautical miles of side. The survey quadrate was ensonified each day four times in the 24 h. In 1997 the acoustic survey was conducted in the same area and in the same week of the year to analyse the diel feeding cycle of herring and sprat and their size distribution by depth and time of the day using pelagic trawls. Fish abundance, from 1995 survey, was statistically different among days and survey quadrates. However, from our data it is not clear whether the variation stems from random dispersion or directed movements occurring at the temporal small-scale. Pelagic fish were dispersed during the night at the surface and aggregated during the day at the bottom. They aggregated at dawn and dispersed at dusk at the surface. For herring this distribution pattern coincided with peaks of stomach fullness analysed in the 1997 survey, while sprat seemed to continue feeding during the whole day time. Larger herring were deeper in the water column than smaller individuals. Diel vertical migrations (DVM) of pelagic fish likely mirrored zooplankton diel vertical movements and it was reasonably in response to optimal predation conditions in the sea and possibly intertwined with predation avoidance and bioenergetic optimisation.

Lower trophic levels were compared in embayment, nearshore, and offshore habitats of Lake Ontario, 1995 to 1997, in the context of oligotrophication and invasion of dreissenid mussels. Total phosphorus (TP), chlorophyll a (chl a), Secchi disk depth, temperature, and zooplankton were measured to spatially and temporally contrast these habitats and to test for recently hypothesized "decoupling" of chl a from TP (lower than predicted chl a per unit TP, consistent with dreissenid mussel grazing). The embayment habitat had higher concentrations of TP and chl a, greater volumetric zooplankton density and biomass, and higher springtime water temperatures than both nearshore and offshore habitats, while overall areal zooplankton biomass was highest in the offshore. Furthermore, concentrations of TP and volumetric zooplankton density in nearshore habitats are now more similar to the offshore pelagia than they were three decades ago. Finally, a lower yield of chl a per unit TP was found in nearshore habitat compared to offshore and embayment habitats. The current lower yield of chl a per unit TP in nearshore habitat can be attributed more to Dreissena than to erosion and/or resuspension of sediments.

The eastern Baltic cod fishery, as many fisheries worldwide, is experiencing a major crisis. The combination of current environmental conditions, fishing mortality rates, and stock size indicates that this fishery is not sustainable. In order to rebuild the population and sustain the fishery in the future, it is essential to define a biologically appropriate exploitation level. In this paper, we take a decision-theoretical approach and use risk analysis to compare different management actions in terms of fishing mortality rates, and predict stock size in a short, medium and long-term perspective. We consider alternative hypotheses about the stock biomass in 1999 and analyse the risk of stock decline below three thresholds: quasi-extinction (B-ext, defined by us), the safe biological limit, and the precautionary. approach limit as defined by the International Council for the Exploration of the Sea (ICES). Results show that the current fishing mortality implies a tremendous risk of driving the Baltic cod stock below B-ext (1000 t) within 10 yr. This result is robust to alternative assumptions about model structure as well as to the magnitude of environmental stochasticity. Hence, we consider a substantial reduction of the fishing mortality to be the only way of avoiding the far-reaching ecological and socio-economic consequences of continued exploitation at current levels.

We analysed spatial and temporal patterns of condition factor (CF) of sprat (Sprattus sprattus) between 1986 and 2000 in different areas of the Baltic proper. Results showed a moderate increase in CF between 1986 and 1989. Afterwards, an abrupt decrease in CF, between 22% and 29%, occurred in all the areas of the Baltic proper. However, from 1998 and onwards, CF increased compared to the previous years. Weight-at-age and CF of Baltic herring showed a similar pattern in the same period. Several factors affecting variability of sprat CF were evaluated. Data showed that sprat CF was density-dependent and possibly related to the pelagic fish abundance and individual food intake. Temperature did not influence sprat CF while salinity seemed to affect variability of sprat CF in the northern Baltic proper. Changes in salinity levels may shape pelagic fish growth rates both indirectly changing the zooplankton community structure and abundance and/or directly via fish physiology and metabolisms. (C) 2002 Ifremer/CNRS/Inra/IRD/Cemagref/Editions scientifiques et medicales Elsevier SAS. All rights reserved.

The recent decline in the Baltic cod Gadus morhua L. has been accompanied by an increase in population variability.Small population size, poor recruitment, and high population variability may be factors contributing to their imminent (commercial) extinction.Here we show that variable harvest rate, rather than variable recruitment, is the plausible cause of the increased variance in Baltic cod stock size.This calls for a reconsideration of the role of harvesting as a driving force in exploited populations.The mean exploitation rate, and also the variance, ought to be taken into account in fisheries and other population management.

The recruitment process and its underlying mechanisms are among the most studied phenomena in fisheries ecology. Traditional models estimate fish recruitment assuming a direct relationship with spawning stock size. However, highly variable environmental conditions, feeding conditions, and other factors can influence and complicate the results of a simple linear regression analysis between stock and recruitment. We used generalized additive models (GAMs) to investigate the influence of environmental conditions and stock structure on the recruitment processes of Baltic cod. The interaction between abiotic factors and old spawners (>5+ years) and eggs produced by old spawners were the most significant explanatory variables. Eggs produced by young spawners have a positive impact on cod recruitment only at high levels of reproductive volume, while old spawners' eggs have the highest positive effect at low levels of reproductive volume. Here we show: (i) that the number of Baltic cod recruits is strictly dependent on the age structure of the population; (ii) that interactions between biotic and abiotic factors are crucial in explaining recruitment variability; and (iii) that GAMs are a powerful technique for defining and quantifying the intricate multidimensional relationship between biotic and abiotic variables involved in recruitment processes.

We used split-beam hydroacoustics (Simrad EY500, 70kHz) as a method for estimating fish swimming speed in situ. The method was first evaluated in the field using underwater video cameras (stereocinematographic method, SCG) to estimate accurate fish swimming speeds. The mean and distribution of swimming speeds of 15-cm brook trout (Salvelinus fontinalis) obtained by the two methods were not statistically different (average 17.8cms–1with split-beam and 18.6cms–1with SCG). We then used the split-beam technique to measure swimming performance in situ for fish assumed to be yellow perch (Perca flavescens) and alewives (Alosa pseudoharengus) in two lakes in New York State, USA. The measured swimming speeds ranged from 0.5 to 6 body length (BL) per second for juvenile and adult fish. Other laboratory studies on swimming speeds have reported values in the same range. However, measured swimming speeds for smaller fish were unrealistically high (2–32 BLs−1) Advantages of the split-beam method are the ability of measuring swimming speed independently of visibility, with minimal disturbance and at large distances. Disadvantages are the inability to distinguish species observed and some variance in target location, which results in calculated average swimming speeds of 2.6cms–1 even for a stationary target.

The management of exploited fish populations is, at present, based on stock assessment methods which assume that spawning biomass is proportional to the reproductive potential of the stock. This implies that the survival rates of offspring do not substantially change with the age or size of the spawner. However, the commercial extinction of many fish stock worldwide has provoked concern about the techniques used in stock assessment. Is the contribution of different age classes to recruitment equal, irrespective of the age of the fish? Here we show that not only do older individuals contribute the largest amount of eggs to the reproductive potential of cod Gadus morhua, but that they also have the largest number of recruits likely to produce offspring with higher rates of survival. This important effect of population structure on recruitment should be taken into account in fisheries science and fisheries management.

Acoustic surveys often involve more than one research vessel and the results are combined to produce an estimate of the abundance of a stock. Intercalibration is a routine procedure carried out on surveys to check that the acoustic systems on the vessels are working properly. To fmd reasons for the differences measured in the past, an inter-vessel comparison was conducted in October 1997 in the southern Baltic Sea between the German RV Solea and the Swedish RV Argos. The results showed that a side-shifted transducer on the RV Solea again gave 1.4 to 2.5 times higher nautical area scattering coefficients (s(A) values) than a hull-mounted transducer used by RV Argos. There is no explanation for this phenomenon at this stage and it has not been possible to calculate a simple conversion factor. However, the noise from the ship was detected as the most significant factor. The fright reaction of the fish depends on the noise level and this depends on several other factors like source level, directivity function of the vessel and che water depth. Since these factors are usually different for different vessels, we can not expect the same value for each vessel. As the transducers used were optimised for different situations the variation in measurements was extremely large. However, the extent of the behavioural reaction exhibited by the fish seems to depend on two combined factors: lateral avoidance and the tilt angle effect of avoidance.

Stock assessment models used in fisheries management require an accurate estimation of the age-structure of the fish populations. This is often derived by ageing a number of individuals (counting rings in sectioned otoliths) and estimating the relative percentage per age class. In this paper, we investigated if a strong relationship between otolith weight and fish age could exist in two important commercial species: plaice (Pleuronectes platessa) and Baltic cod (Gadus morhua). A highly significant relationship was shown between otolith weight and fish age in both species and otolith weight was independent of fish length. The latter has important implications because it means that otolith weight is a much stronger predictor than fish length and it can be used to estimate fish age. The relative age-structures estimated using otolith weight were not significantly different from those estimated by traditional annuli counting in both species. For each new sample of fish, the linear regression between otolith weight and fish age needs to be confirmed in order to use the same parameters for the estimation of fish age using the otolith weight. However, the necessary recalibration was easily performed using a 100-individuals random sample. Therefore, we suggest that otolith weight could represent a valuable criteria for an age determination technique that is objective, economic and easy to perform compared to traditional methods that have been defined “as much an art as a science”.

Observations during the 1990s indicate that individual growth rate of Atlantic herring (Clupea harengus) decreased by 30% to 50% in the Baltic proper. There are several possible explanations, however, no statistical analysis has been performed to define and describe the changes in herring growth rates. Tn this study we compared mean weight-at-age and length-specific weight (i.e. condition) data collected in September-October 1986-1996. Results indicated an increase in weight-at-age between 1986-1989. Thereafter, weight-at-age and fish condition declined in all parts of the Baltic proper. The decreases were evident in 0 through to 8 year old fish. Moreover, the declines were larger in the northern (51%) than in the south-western (42%) part of the Baltic proper and for younger (1-4 years) fish compared to the older fish (5-8 years). Possible causes of the decrease in weight-at-age and condition are discussed. However, the decreases seem to be well correlated with a drastic increase in the number of pelagic fish in the Baltic proper and a consequent reduction in food availability. (C) 2000 International Council for the Exploration of the Sea.

For the estimation of the stock biomass measured during hydroacoustic surveys it is necessary to take fish samples. These samples should represent the length distribution and species composition of the fish estimated by the sounder. Therefore the trawl has to catch every object in the volume of water sampled. To approximate this, the selectivity of the used trawl should be low. However, specially designed trawls are rarely available. Usually modified commercial trawls are used. Two research vessels from Sweden and Germany carried out a co-ordinated survey, to investigate trawl catch composition of high fish densities scattered layers of herring and sprat in the Baltic Sea. The analysis indicated that the selectivity for young fish was different between trawls because of the different mesh sizes in the codend. Thus, there is a need to estimate the real size distribution. A correction function was designed by applying selectivity data obtained from commercial trawls. However, these corrections are only possible for data obtained with mesh sizes not much larger than the optimum size. Results of this study also indicated that the larger the trawl, the larger the proportion of larger fishes in the length distribution. This effect could not be explained by selectivity. Thus further research is necessary. In every case it is advisable to check the selectivity of the sampling trawl in the described way.

We have determined the in situ food consumption rate of young-of-the-year (YOY, length 25 to 94 mm) Baltic Sea herring. This consumption increased with increasing food availability, which suggests that the growth rate of young Baltic herring is food Limited. This has been described previously for larval stages, but not for metamorphosed fish. The daily food consumption rate of YOY herring decreased from about 8 to 15% of body weight for small fish in the summer to 0.6 to 2 % for larger juveniles in late autumn.