Constant catch or constant harvest rate?: The Baltic Sea cod ( Gadus morhua L.) fishery as a modelling example

Abstract

Overfishing is a major problem in the world fisheries. Constant harvest rate strategies, with catches proportional to the abundance of the target species, stimulates investments when stock sizes are large, often resulting in an overcapacity when the stocks decrease. These investment incentives are weaker under a constant catch strategy, since there are less reasons to have a catching and fish processing capacity that is larger than the allowed constant catch. We have explored the potential of a management strategy based on constant catches, by modelling the fishery on the eastern Baltic Sea cod ( Gadus morhua L.). To avoid the low long-term yield (LTY) often resulting from a strict constant catch approach, we developed a quasi-constant catch (QCC) strategy based on alternative constant catch levels and two overfishing criteria. Normal catches are allowed when the spawning stock biomass is above a certain level. When the spawning stock biomass falls below this level, catches are reduced to a given extent. If the spawning stock biomass falls below a second and lower threshold, the fishery is closed. The QCC strategy was tested with a model based on the same basic premises as the multi-species virtual population analyses (MSVPA) for Baltic Sea cod, developed within ICES. In addition, recruitment processes were included and probability analyses were done to estimate different risks of overfishing. According to the model, the LTY with the QCC strategy was 10% lower than with the constant harvest rate strategy. However, the QCC approach had the advantage of smaller variation in catches, potentially resulting in a better utilisation of the fishery capacity. The capacity utilisation was better with the QCC strategy relative to the constant harvest rate strategy, especially in fisheries where catch quotas are based on assessments with substantial errors. Furthermore, QCC gave a better capacity utilisation in fisheries where the size of the fleet and processing capacity is non-flexible and when effective fish finding techniques are used. A more effective capacity utilisation could counterbalance the disadvantage of the lower LTY compared to the constant harvest rate strategy. The QCC approach is hence an interesting and competitive management strategy, that is likely to be more effective in avoiding the emergence overcapitalisation and overfishing of long-lived species like the Baltic cod. QCC may also allow catch quotas to be set for more than 1 year, provided that the stock size of the target species is relatively large.