W.L.T. van Densen

Effect of snood length and hanging ratio on efficiency and selectivity of bottom-set gillnets for pikeperch, Stizostedion lucioperca L., and bream, Abramis brama

Abstract Experiments were conducted with multi- and monofilament bottom-set gillnets of various mesh sizes (50, 55 and 65 mm bar mesh), snood length (0.6 and 1.1 m) and hanging ratio, E (0.25, 0.33, 0.5) to study the influence of gear construction on the selectivity of the nets for catching pikeperch ( Stizostedion lucioperca ) and bream ( Abramis brama ). Catches were always higher in the conventional gillnets with 1.1 m snoods, but on an exposed net area basis, catches of bream were higher in the nets with 0.6 m snoods. Catches in the gillnets with the lowest hanging ratio ( E = 0.25) were always higher than in the conventional gillnets ( E = 0.50). Catches of pikeperch were higher in the multifilament nets, those of bream were higher in the monofilament nets. Lowering the hanging ratio in the conventional nets to 0.25 always resulted in a higher mean length of fish caught.

Comparison of methods for the estimation of gillnet selectivity to tilapia, cyprinids and other fish species in a Sri Lankan reservoir

Methods based on the models of Holt (1957) and Sechin (1969a, b) were compared for the estimation of gillnet selectivity for two tilapiine, five cyprinid, two catfish, one goby and one halfbeak species, inhabiting a Sri Lankan reservoir. Holts model was extended to the case where the standard deviation of the selection curve increases in proportion to the optimum selection length. Sechins model was extended to the case where standard deviation of body girth increases in proportion to the body girth. Selection curves from extended Holt and Sechin models were compared with length frequency distributions of the catch (catch LFD) for the largest samples per species. A method for rapid estimation of the position and shape of selection curves was proposed. This method is based on the most common position of retention in gillnets, the length-girth relationship at that position and the ratio between mesh perimeter and body girth. A constant coefficient of variation of 9% was considered to be a good estimate for the standard deviation of the selection curves for non-tangled fish. Tangling percentages were low for all but one toothy species, Glossogobius giuris (Hamilton), which showed bimodality in the catch LFD owing to high tangling percentages. The selectivity for different species was compared using the girth at the position of retention as the independent variable. Differences between species were small and a rough estimation for the most efficiently selected retention girth could be calculated as 2.10 times the stretched mesh size, or 1.05 times the mesh perimeter, independent of fish species. The Holt and Sechin models are only suitable to reconstruct population structures of species with low tangling percentages. The extended Holt model is preferred over the Sechin model, because the Sechin model results in selection curves which are systematically more narrow than the catch LFD and Holt selection curves.

Trends in an Indonesian coastal fishery based on catch and effort statistics and implications for the perception of the state of the stocks by fisheries officials

Indonesias capture fisheries are monitored in each district of all 27 provinces with a comprehensive catch and effort data recording system that was installed in 1976. The annual data are sent to the Indonesian Directorate General of Fisheries (DGF) in Jakarta, where these are aggregated for nine regions. Catches for the 45 recognised fish categories are accumulated in four major fish groups and analysed with conventional fisheries surplus models to estimate maximum sustainable yields (MSYs). These estimates have been used by DGF to determine the number of fishing licenses for each region in the nations economic exclusive zone (EEZ). This paper discusses the effect of data aggregation and accumulation on the variance around trends in fisheries data of the South Sulawesi province. Simple regression techniques are applied to time series of catch, effort, catch-per-unit-effort, and numbers of boats. At the lowest level of data aggregation and accumulation we find the highest variance. Although high variance obscures the perception on the state of fish stocks at the lowest levels, perceptions at the highest levels are not necessarily more useful for fisheries management. Bias caused by motorisation of the fleet by using CpUE as an indicator of fisheries mortality and by combining data from administrative units that have no ecological or biological meaning obscures the detection of trends.

A size- and age-structured simulation model for evaluating management strategies in a multispecies gill net fishery

Abstract A size- and age-structured simulation model is presented for evaluation of management strategies for a multispecies gill net fishery, using a commercial gill net fishery for pikeperch (Stizostedion lucioperca) and perch (Perca fluviatilis) as an example. The model takes size distributions within age groups into account. Growth for each sex is determined by length and temperature and dispersion in size distributions is controlled by the ‘fractional boxcar train’ method. The model is applied to evaluate the integrated short-term and long-term effects of management measures such as mesh size regulations, fishing effort limitations and combinations of both on the biomass, the size- and age-structure of the stock and of the yield. Changes in yield during the transitional period after a management measure has been implemented, can be quantified.

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.

Catch, effort and sampling strategies in the highly variable sardine fisheries around East Java, Indonesia.

Abstract Temporal and spatial patterns in the fishery for Sardinella spp. around East Java, Indonesia, were studied in an attempt to develop an efficient catch and effort sampling strategy for this highly variable fishery. The inter-annual and monthly variation in catch, effort and catch per unit of effort (cpue) are quantified and discussed, using official catch reports and reports from local village cooperatives. In 1993 the total catch of Sardinella spp. in East Java was 56 000 tonnes and the total effort was equivalent to 5000 units of purse seine. A maximum sustainable yield (MSY) of 40 000 tonnes of sardines is estimated, at an optimum effort of 3000 units of purse seine. The present study focuses on the purse seine fishery since this fishery accounts for more than 50% of all sardines landed in East Java. Purse seiners use lamps to attract fish and peak catches are landed around the new moon. Catches are smaller towards the full moon, when the fishery is inactive during a period of 10 days. Considerable differences exist between monthly catches from peak seasons and monthly catches from off-seasons. The fishery is further characterized by a relatively large amount of zero catches. A catch and effort data recording system (CEDRS) is developed based on the analysis of temporal variation in cpue and effort in the purse seine fishery for sardines. A purse seine fleet, with a variable size of 40 to 70 vessels, was sampled in Probolinggo harbour at the Madura Strait, during 4 to 6 days per month from March 1990 until March 1991. The overall mean effort of the fleet was 36.1 trips per day, of which on average 12.6 trips per day resulted in non-zero catches. The geometric mean cpue over the non-zero catches was 404 kg per trip. The proposed CEDRS has an accuracy of 30% for the estimated annual catch, 21% for the cpue and 9% for the effort (non-zero catches). A minimum required number of 4 sampling days per month is calculated for the temporally stratified CEDRS, with months used as strata within years, and lunar phases as strata within months. For this purpose, the lunar cycle is divided in three lunar phases, one around the full moon, one around half moon and one around the new moon. Within the proposed CEDRS, no sampling takes place in the lunar phase around full moon, when the fishery is inactive. The sample of 4 days per month is proportionally allocated over the other two lunar phases.

Length-based analysis of population dynamics and stock identification in the sardine fisheries around East Java, Indonesia

Abstract The biology and population dynamics of Sardinella longiceps and S. fimbriata were studied around East Java, Indonesia. The total catch of Sardinella spp. in East Java was 56 000 tons in 1993, and more than 50% of this catch was landed by purse seiners. Samples of purse seine catches were taken from three landing sites around East Java, at the Madura Strait, the Bali Strait and at the Indian Ocean. Biological data were collected on a monthly basis, during a period of 2 years, from March 1990 to March 1992. Patterns in reproductive activity were determined on the basis of changes in geometric mean gonado-somatic index (GSI) of mature fish. Length-based methods were used to estimate growth and mortality. Peak catches of sardines from the Madura Strait and the Bali Strait were recorded at the beginning of the rainy season around November and December. The Indian Ocean produced peak catches during the dry season, from July through October. Considerable differences exist between monthly catches from peak seasons and monthly catches from off-seasons. Patterns in reproductive activity could be recorded for the fish from the Madura Strait and both species of Sardinella showed peak reproductive activity there around November and December. Both species were estimated to grow to lengths of around 12, 17 and 19 cm in their first, second and third year respectively. Based on the age structures in the catches from different sampling sites, in combination with the migration patterns of the fish, it was concluded that both S. longiceps and S. fimbriata stocks can be divided into northern and southern populations. The northern populations were sampled in the Madura Strait and were assumed to be from the Java Sea, Makassar Strait and/or Flores Sea. These stocks consist of three year classes and do not seem to be under excessive fishing pressure. The southern stocks, well known from the Bali Strait sardine fisheries, are concluded to be under very heavy fishing pressure. These stocks consist of only two year classes and hardly any fish reach the length of maturity. Regulations controlling fishing effort are needed urgently for these fisheries.