Changes in seawater nutrient concentrations during purse seine fishing for sardine Sardina pilchardus off northern Portugal

Abstract

Surface water samples were collected before, during and after purse seining for sardine off northern Portugal (12 sets) to test the hypothesis that fishing can lead to changes in local nutrient concentrations. Ammonia, urea, residual organic nitrogen and phosphate concentrations signifi- cantly increased during fishing and the increase was a pulse event associated with the drying-up of the net (when fish density in the net is highest). Concentrations at the end of fishing (on average 40 min after drying-up) were lower than peak values but still significantly higher than the original concentrations. Nitrate and nitrite did not alter during the fishing operation, further suggesting that the observed increase in nitrogen and phosphorus compounds is of biological origin. There was no significant relation between peak concentrations and fish catch, suggesting that the fisheries- associated stimulus does not affect equally all the fish in the net (relative position in the net and fish condition may influence the probability of release). Published data on sardine nitrogen excretion and observations on nutrient concentrations in sardine body parts indicate that enhanced excretion of nitrogenous waste stimulated from stress reactions could account for the observed increase in ammo- nia and urea. Skin mucus release from scale loss (occurring extensively during the drying-up) is the most likely source of phosphorus, while forced evacuation of partly digested food items from the intestine or regurgitation from the stomach (both associated with the mechanical pressure and contact among fish and with the net walls at the end of drying-up) probably also contribute to the increase in residual organic nitrogen.