Water quality and rainfall conditions were analysed from November 1996 to September 1997 in selected sampling sites in the Bellanwila±Attidiya wetlands of Sri Lanka, an area where the epizootic ulcerative syndrome (EUS) has occurred, in order to examine the environmental factors that may be associated with EUS. Quantitative sampling of ®sh populations was carried out over the study period and the recurrence of EUS was monitored by gross clinical signs and histopathology. The occurrence of EUS from the sampling sites was recorded from January 1997 to March 1997. The outbreak was preceded by heavy rainfall. Diurnal variations in air temperature were relatively high during the outbreak period. Declining dissolved oxygen concentrations in water coincided with initiation of the disease outbreak. There were signi®cantly low levels of dissolved oxygen in the water and signi®cantly low rainfall in the area during the period of EUS in comparison with non-EUS periods. The biochemical oxygen demand of the water was signi®cantly higher during the outbreak period than that in the period prior to the outbreak. Of the ®sh species sampled, the prevalence of EUS was highest in Trichogaster pectoralis (snake skin gourami). Histopathological studies on ulcerative lesions of ®sh con®rmed that diuse proliferative mycotic granulomatosis is a consistent feature of EUS. The results indicate that interaction between rainfall, deteriorating water quality and presence of pathogens could provide stressful conditions for ®sh, thereby inducing EUS lesions in susceptible ®sh populations.

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The eggs of Polka-dot ribbonfish Desmodema polystictum have been recorded for the first time in the Indian Ocean. Although the only previous information on eggs from this species consists of line drawings from 1973 (Pacific Ocean), the identification was possible by combining morphological and molecular analyses. As far as we are aware, only few confirmed records of adult individuals have been previously reported from the Indian Ocean. We found eggs in the proximity of numerous adults (57 and 42 individuals at two stations) indicating that the Central Indian Ocean is potentially an important spawning ground of D. polystictum.

As in many tropical reservoir fisheries, the major fishing gear in the reservoirs of Sri Lanka is gillnet. Gillnets of a wide range of stretched mesh sizes (6.9 -11.4 cm) are used in individual boats in Sri Lankan reservoirs targeting mainly two exotic cichlid species, Oreochromis mossambicus and O. niloticus, which dominate the fisheries accounting for over 80% of the landings. Although the filament characteristics and dimensions of gillnets of different mesh sizes are uniform, their mesh composition varies from boat to boat making catch samples in individual boats are under the influence of different selectivity patterns. As such, an approach is presented for constructing the overall selection curves in the sampled boats during different sampling occasions for the two cichlid fish species in the three reservoirs. For this purpose, gillnet selectivity of each mesh size was determined using Baranov-Holt method. Probabilities of capture of meshwise gillnet selection curves were then summed up giving weight to the contribution of each mesh size to the total number of net pieces in the sampled boat, to determine the overall gillnet selection from gillnets of all mesh sizes. The importance of the findings of the present study for length-based stock assessment methods and for imposing mesh regulations to the gillnet fisheries in reservoirs of Sri Lanka is discussed.

No abstract

Although vital in fisheries management, no comprehensive studies on ichthyoplankton have been conducted in Sri Lankan waters in the north central Indian Ocean hitherto. Hence, this study is the first detailed account of diversity and species composition of larval fish based on samples collected during the southwest monsoon in 2018. In total, 80 species belonging to 69 families were identified using morphological and molecular methods targeting the COI gene. The larval fish diversity varied significantly between regions: east (North East, Central East, and South East) and west (North West, South West, and South). In their larval stages, mesopelagic families were associated with the offshore waters whereas demersal and pelagic families were related with shelf regions in the South, South East, and Central East. The larvae of pelagic families are likely dispersed by the South Monsoon Current from the west to the east regions, while demersal fish seem to be confined to the same area as conspecific adults. The most abundant larval species observed were Selar crumenophthalmus, Cubiceps pauciradiatus, and Dipterygonotus balteatus. High abundances of several commercially important larval tuna species were found in the South East, Central East, and South regions indicating that these waters could be important nursery grounds. Furthermore, Callionymus simplicicornis was recorded for the first time in the Indian Ocean, and seven additional species were found new to Sri Lankan waters. The results from this study also highlight the importance of using combined morphological and molecular methods and the need for strengthening fish nucleotide databases in poorly studied areas of the Indian Ocean.

European marine waters include four regional seas that provide valuable ecosystem services to humans, including fish and other seafood. However, these marine environments are threatened by pressures from multiple anthropogenic activities and climate change. The European Marine Strategy Framework Directive (MSFD) was adopted in 2008 to achieve good environmental status (GEnS) in European Seas by year 2020, using an Ecosystem Approach. GEnS is to be assessed using 11 descriptors and up to 56 indicators. In the present analysis two descriptors namely "commercially exploited fish and shellfish populations" and "food webs" were used to evaluate the status of subareas of FAO 27 area. Data on life history parameters, trophic levels and fisheries related data of cod, haddock, saithe, herring, plaice, whiting, hake and sprat were obtained from the FishBase online database and advisory reports of International Council for the Exploration of the Sea (ICES). Subareas inhabited by r and K strategists were identified using interrelationships of life history parameters of commercially important fish stocks. Mean trophic level (MTL) of fish community each subarea was calculated and subareas with species of high and low trophic level were identified. The Fish in Balance (FiB) index was computed for each subarea and recent trends of FiB indices were analysed. The overall environmental status of each subarea was evaluated considering life history trends, MTL and FiB Index. The analysis showed that subareas I, II, V, VIII and IX were assessed as "good" whereas subareas III, IV, VI and VII were assessed as "poor". The subareas assessed as "good" were subject to lower environmental pressures, (less fishing pressure, less eutrophication and more water circulation), while the areas with "poor" environment experienced excessive fishing pressure, eutrophication and disturbed seabed. The evaluation was based on two qualitative descriptors ("commercially exploited fish and shellfish populations" and "food webs") is therefore more robust.

Amarasinghe U.S., Jayasinghe R.P.P.K., Moreau J. 2017. Length-based stock assessment of Oreochromis mossambicus and Oreochromis niloticus (Actinopterygii: Perciformes: Cichlidae) in multi-mesh gillnet fisheries in reservoirs of Sri Lanka. Acta Ichthyol. Piscat. 47 (3): 265-277.Background. Gillnets of mesh sizes (6.9 to 12.7 cm) are used in individual boats in Sri Lankan reservoirs targeting exotic cichlid species, Oreochromis mossambicus (Peters, 1852) and Oreochromis niloticus (Linnaeus, 1758), which dominate the fisheries. The mesh composition of gillnets varies from boat to boat and from season to season making catch samples in individual boats are under the influence of different selectivity patterns. As such, the conventional procedure of correcting length frequency samples using the overall mean selectivity pattern in the fishery is not precise in estimating growth parameters. The presently reported study aimed at assessing cichlid stocks in three tropical reservoirs accounting for gillnet selectivity of individual sampled boats. Material and methods. Length frequency data obtained from each boat were corrected for the combined selectivity of the fleet of gillnets of different mesh sizes used in the boat during the sampling occasion. For predicting optimal fishing strategies using a yield-per-recruit approach, probabilities of capture determined from the array of fishing mortality in the length-structured virtual population analysis was incorporated.Results. Length frequency data corrected for overall gillnet selectivity in sampled boats gave reliable estimates of von Bertalanffy growth parameters. Based on these estimates, mortality parameters were determined and the overall gear selection pattern was deduced from the array of fishing mortality in the length-structured virtual population analysis. Optimal fishing strategies were determined incorporating overall probabilities of capture in the relative yield-per-recruit (Y′ × R -1 ) analysis. Conclusion. The overall gillnet selection pattern of a sampled boat offers a methodology for reliable estimation of growth parameters. Although both cichlid species are caught in gillnets simultaneously, Y′ × R -1 analyses predicted increase of exploitation rate for one species while its decrease for the other species perhaps due to influence of other factors such as fish behaviour and fishers' choice of specific areas for fishing. This analysis provides a methodology of a more precise length-based stock assessment in future research.