Fredrik Arrhenius

Diel spatial distribution and feeding activity of herring ( Clupea harengus) and sprat (Sprattus sprattus) in the Baltic Sea

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.

Potential use of otolith weight for the determination of age-structure of Baltic cod (Gadus morhua) and plaice (Pleuronectes platessa)

Abstract 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”.

Comparison of the selectivity of three pelagic sampling trawls in a hydroacoustic survey

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.