The impacts of increased codend mesh size on the northern Baltic herring fishery: ecosystem and market uncertainties

Abstract

Kuikka, S., Suuronen, P., and Parmanne, R. 1996. The impacts of increased codend mesh size on the northern Baltic herring fishery: ecosystem and market uncertainties.-ICES Journal of Marine Science, 53: 723-730.The short and long term effects of an increase in trawl codend mesh size (from 20-mm to 36-mm) on catch weight and value per recruit and on the stock biomass of herring in the northern Baltic were assessed. Calculations were carried out using an agestructured model. Yearly estimates of growth rates for commercial catch sample data for 1974-1992 were used in the analysis. Natural mortality estimates were from the multispecies population analysis. Under the conditions prevailing in 1974-1992, the increase in codend mesh size would have led to a reduction in catch and in the value of the catch per recruit. The magnitude of the estimated reduction in catch varied greatly with the growth and natural mortality of the population. If an increase in mesh size is to be profitable for this fishery in the long run, the price of large herring processed for human consumption would have to be six times greater than the price of the smaller fodder herring (the price differential has been around four-fold), or the survival of codend escapees would need to be increased to 80% from the current estimated level of about 15% (e.g. with the aid of new fishing technology).1996 International Council for the Exploration of the Sea

Assessing the effects on stocks of selectivity changes in a fishery

“…Consequently, for this stock, relatively small adjustments in fleet selectivity can improve the fishing pattern considerably. Kuikka et al (1996) studied the impacts of increased codend mesh size on the northern Baltic herring fishery with an age-structured population model. Their selection curves had to be transformed into curves which described retention by age.…”

“…By using age-length structured population models, however, growth variation is handled directly. Kuikka et al (1996) assessed the economic value of the annual catches in addition to biomass-which could also be included in our simulations.…”

Escape mortality of trawl caught Baltic cod (Gadus morhua) — the effect of water temperature, fish size and codend catch

“…fishres.2004.08.022 there may be no advantages of increasing the mesh size (e.g. Kuikka et al, 1996;Breen and Cook, 2002). In the worst case the effect of this type of unaccounted mortality may be negative on fish stocks because the overall mortality caused by the exploitation is underestimated.…”

Unaccounted mortality in northern Baltic Sea herring fishery—magnitude and effects on estimates of stock dynamics

“…In addition, mesh size regulations were planned so that only the use of the 36 mm codend would be allowed, the target being to cease fishing for animal fodder while ensuring supply to human consumption. Such plans were, however, rejected as it has been shown that increased codend mesh size would reduce the value of catch per recruit due to low survival of the escapees (Kuikka et al, 1996). Although the impact of unaccounted mortality on the standard stock assessment output is limited, it poses certain problems for the assessment procedure.…”