J.J. Poos

In search of a better unit of effort in the coastal liftnet fishery with lights for small pelagics in Indonesia

Abstract Despite major criticism, catch per unit effort (CPUE) is still widely used as a measure for the size of the exploited stock, but its indicative value is affected by selection of a proper unit of effort. The unit of effort used in the Indonesian fisheries statistical system is poorly standardised with one trip made by a fishing unit of any type. Therefore, changes in catchability, induced by technical innovations cannot be accounted for in official statistics and thus bias the perception of fisheries authorities on the state of stocks. This study shows that differences in technical characteristics in a coastal liftnet fishery on small pelagics around Ambon, such as unit size and type of lamps used to attract fish, influence both effort allocation and catch per trip. Larger fishing units (more windlasses) are more commercially orientated. They have a larger nonzero catch per trip—1.5 times larger nonzero catch per added windlass—and ignore potentially small catches, as shows from their higher proportion of zero-catches. The shift from kerosene to electric lamps does not lead to a higher catch per trip, but the use of electric lamps is less laborious, inviting fishermen to fish also under more unfavourable light conditions such as during full moon. Model simulations of average catch per trip of two fishing areas show that technical innovations in the last 20 years have increased the mean catch per trip by a factor of 4.1 in the fishing area with the largest technical improvements. Technical improvements explain 80% of the change in catch per trip in the official statistics over the last 15 years. Furthermore, the variance in catch per trip among individual fishing units has increased and so has the uncertainty in the average CPUE, as only part of the fishermen implemented technical improvements. Both developments stress the poor indicator value of catch per trip for developments in fish stock biomass and the importance to standardise the unit of effort. The characteristic high variability in catch in this fishery will always obscure effects of technical improvements in more local situations and smaller timeframes. Because of the inconsistent effects of the type of lamps used on the catch rate, it is questionable whether standardising the unit of effort by the type of lamp used will enlarge the precision in the CPUE. Nevertheless, standardising the unit of effort by the size of the unit will enlarge the precision in, and the indicator value of the CPUE in more aggregated catch data as used on higher levels in the fisheries administration of Indonesia.

Interspecific resource competition effects on fisheries revenue.

In many fisheries multiple species are simultaneously caught while stock assessments and fishing quota are defined at species level. Yet species caught together often share habitat and resources, resulting in interspecific resource competition. The consequences of resource competition on population dynamics and revenue of simultaneously harvested species has received little attention due to the historical single stock approach in fisheries management. Here we present the results of a modelling study on the interaction between resource competition of sole (Solea solea) and slaice (Pleuronectus platessa) and simultaneous harvesting of these species, using a stage-structured population model. Three resources were included of which one is shared with a varied competition intensity. We find that plaice is the better competitor of the two species and adult plaice are more abundant than adult sole. When competition is high sole population biomass increases with increasing fishing effort prior to plaice extinction. As a result of this increase in the sole population, the revenue of the stocks combined as function of effort becomes bimodal with increasing resource competition. When considering a single stock quota for sole, its recovery with increasing effort may result in even more fishing effort that would drive the plaice population to extinction. When sole and plaice compete for resources the highest revenue is obtained at effort levels at which plaice is extinct. Ignoring resource competition promotes overfishing due to increasing stock of one species prior to extinction of the other species. Consequently, efforts to mitigate the decline in one species will not be effective if increased stock in the other species leads to increased quota. If a species is to be protected against extinction, management should not only be directed at this one species, but all species that compete with it for resource as well.

Harbour seals are regaining top-down control in a coastal ecosystem

Historic hunting has led to severe reductions of many marine mammal species across the globe. After hunting ceased, some populations have recovered to pre-exploitation levels, and may again act as a top-down regulatory force on marine ecosystems. Also the harbour seal population in the international Wadden Sea grew at an exponential rate following a ban on seal hunting in 1960’s, and the current number ∼38,000 is close to the historic population size. Here we estimate the impact of the harbour seal predation on the fish community in the Wadden Sea and nearby coastal waters. Fish remains in faecal samples and published estimates on the seal’s daily energy requirement were used to estimate prey selection and the magnitude of seal consumption. Estimates on prey abundance were derived from demersal fish surveys, and fish growth was estimated using a Dynamic Energy Budget model. GPS tracking provided information on where seals most likely caught their prey. Harbour seals from the Dutch Wadden Sea fed predominantly on demersal fish, e.g. flatfish species (flounder, sole, plaice, dab), but also sandeel, cod and whiting. Total fish biomass in the Wadden Sea was insufficient to sustain the estimated prey consumption of the entire seal population year-round. This probably explains why seals also acquire prey further offshore in the adjacent North Sea, only spending 13% of their diving time in the Wadden Sea. Still, seal predation was estimated to cause an average annual mortality of 43% and 60% on fish in the Wadden Sea and adjacent coastal zone, respectively. There were however large sources of uncertainty in the estimate, including the migration of fish between the North Sea and Wadden Sea, and catchability estimates of the fish survey sampling gear, particularly for sandeel and other pelagic fish species. Our estimate suggested a considerable top-down control by harbour seals on demersal fish. However predation by seals may also alleviate density-dependent competition between the remaining fish, increasing fish growth, and partly compensating for the reduction in fish numbers. This study shows that recovering coastal marine mammal populations could potentially become an important component in the functioning of shallow coastal systems.