Influence of thermal conditions on biomass of fish in the Polish EEZ

Abstract

Systematic acoustic surveys of fish resources in the Polish EEZ, carried out during the autumn as a part of international ICES programme, begun in 1989. A noticeable increase in fluctuations of fish biomass and water temperature has been observed since 1996. Methods elaborated in Sea Fisheries Institute were applied to correlate acoustic data on fish distribution with relevant environmental factors, and to develop models of trends observed. Relationships between mean temperature at fish depth and total fish biomass in each year were estimated separately for day- and night-time. High positive correlation ( r=0.910 for day-time, r=0.885 for night-time) was found for data from the 1989 to 2000 period. It was shown that fish reaction to temperature was stronger during the night (exponential model), at which time a 3 °C increase of temperature at fish depth could result in a 1.7 multiplier of total biomass in the area. During the day, a 6 °C increase of temperature was necessary to produce a comparable effect. Research was implemented by application of macrosounding methods to confirm direct correlation between temperature and fish distribution along most representative transects in the area. Analysis of all results indicates the role of the surface waters, where temperature seems to be one of the most important factors influencing final geographic fish distributions. Lower temperatures in the Polish EEZ accelerate northeast migration of fish from the zone, reducing a feeding season. It can be concluded also that climate instability can strongly affect temporal distribution of fish. In a longer period such a negative phenomenon can produce a systematic degradation of fish stocks, what could be observed on the basis of our results.