Small-mesh Gillnets Research Articles

Sri Lankan reservoir fishery: a case for introduction of a co‐management strategy

The inland fisheries of Sri Lanka are essentially artisanal on most of the reservoirs in the country. The annual inland fish production declined dramatically after 1990, when state patronage for the development of the inland fisheries was discontinued for 4 years. This decline was shown to be a result of growth overfishing of the two dominant cichlid species which accounted for over 90% of landings. This was a result of using small mesh ( < 6.9 cm) gillnets in the absence of the State‐sponsored monitoring procedure in the fishery after 1990. This indicates that it is necessary to monitor inland fisheries management in Sri Lanka through a centralized authority in the current situation. However, in some Sri Lankan reservoirs, fishing communities can be categorized as ‘organized’ because they collectively make decisions to define procedures for the rational exploitation of the fishery resources. In reservoirs with ‘organized’ fishing, the communities themselves have developed mechanisms to regulate the landing sizes of dominant cichlid fish species through community‐based fisheries management strategies. In such reservoirs, over‐exploitation of fish stocks was not evident, even after 1990, when state‐sponsored monitoring procedures were suspended. Based on these observations, an alternative approach is recommended for the management of Sri Lankan reservoir capture fisheries in which the Government and resource‐users have equal responsibilities in the management of the resources.

The production and trophic ecology of shallow-water fish assemblages in southern Australia I. Species richness, size-structure and production of fishes in Western Port, Victoria

Fishes were sampled by seine and gillnet in three different habitat types (seagrass, unvegetated sediment and channel) within Western Port, Victoria, between 1989 and 1990. Approximately twice as many fish species were collected per seine haul from seagrass habitat ( x ̄ = 8.7 ) compared with intertidal unvegetated habitat ( x ̄ = 4.7 ), while relatively few fish species were collected per haul from deeper unvegetated-channel habitat ( x ̄ = 2.3 ). Most fishes associated with seagrass beds were widely distributed within the embayment, with the notable exception of a number of coastal species that occurred in low numbers at a single site (Cowes Bank seagrass) near the entrance to the bay and had presumably drifted as pelagic larvae from reefs outside. Small fishes (<10 g weight) were more abundant in seagrass than in unvegetated habitats; however, relatively few of these animals were juveniles of commercially fished species. Seagrass beds in Western Port were not found to provide a significantly more important nursery habitat for commercial species than unvegetated habitat. The production of small fishes at different sites was highly correlated with the production of epifauna, particularly crustaceans. We postulate that these factors are causally linked. This hypothesis is supported by seasonal data, which indicate that when the production of crustaceans was at its lowest during May, populations of small fishes were rapidly declining in Western Port, either by dying or emigrating. Seagrass beds supported over twice the production of small fishes as unvegetated habitat (3.82 g · m −2 · yr −1 cf. 1.58 g · m −2 · yr −1 in unvegetated habitat). Because an estimated 178 km 2 of seagrass habitat has been lost from Western Port since 1973, the reduced production of fishes in unvegetated habitat translates to a decline in small fish production of 6̃30 tonnes AFDW per year. However, falling catches of only two commercial fishes, six-spined leatherjackets ( Meuschenia freycineti) and blue rock whiting ( Haletta semifasciata), were clearly associated with seagrass loss, with most of the loss of fish production occurring amongst small non-commercial species, particularly syngnathids, clinids and scorpaenids. Differences in production of small fishes between habitats were not reflected higher in the food chain. Although greater numbers of fishes were collected using small-mesh (64 mm) gillnets from seagrass than unvegetated habitats, these differences were due to variation in the catch of the omnivorous mullet Aldrichetta forsteri, the most abundant species, rather than variation in the abundances of piscivorous fish species. No differences were found between catches of fish using large-mesh (108 mm) gillnets placed in different habitats.

Prevalence and abundance of helminth parasites in an intensively fished population of brook trout (Salvelinus fontinalis) at a small subarctic lake

Brook trout from a 5-ha lake near Schefferville, Quebec, were intensively sampled by small-mesh gill nets during 2 successive years to study patterns of helminth parasite occurrence in relation to changes in the age and size class structure of the fish population. Nearly 1800 brook trout were removed from the lake, of which all were measured and weighed, 568 were aged by otolith readings, and 361 were examined for parasites. The digeneans Crepidostomum farionis, Phyllodistomum umblae, and Diplostomum sp. and the cestode Eubothrium salvelini were present in more than 50% of the necropsied fish, and the prevalence and (or) abundance of all species except C. farionis increased from the first to the second year of the study. The fish catch in the first year was dominated by the 3+ year class and older fish, whereas by the second year 1+ and 2+ fish were proportionately more abundant, and the importance of older cohorts declined. This change in brook trout population structure, resulting from gill net selectivity, appears to have had the effect of improving per capita food availability for members of the residual stock. After the initial year of intensive fishing, the fish that remained fed more upon both benthic and planktonic invertebrates and their acquisition of parasites was intensified.

Utilization by fishes of shallow, seagrass-covered banks in Florida Bay: 1. Species composition and spatial heterogeneity

Species composition and relative capture rates of water column fishes occurring on the shallow (<1 m), seagrass-covered mudbanks of Florida Bay were assessed using small-mesh gillnets. The fauna was largely temperate, with few tropical representatives, and was similar to the fish community in adjacent basins. There was a high variability in the catch across the Bay, reflecting heterogeneity in both the physical environment and various aspects of the seagrass canopy. The Gulf site, in the northwestern section of the Bay, had the highest species richness and highest capture rates of individual species, relative to other sites. Higher densities of potential prey, greater available foraging area, and organically rich, fine sediments are probably influential in the greater fish utilization of this bank. The greater exchange of western Florida Bay with open Atlantic or Gulf waters is proposed as a secondary factor influencing species richness; the probability of non-resident species occasionally appearing on western banks is greater than in isolated interior sections of the Bay.

Biomass Dynamics of Lake Superior Lake Herring (Coregonus artedii): Application of Schnute's Difference Model

Schnute's (1985. Can. J. Fish. Aquat. Sci. 42: 414–429) difference model was used to study the biomass dynamics of lake herring (Coregonus artedii) in Thunder Bay and Black Bay, Lake Superior, during 1948–79. Recruitment to both fisheries appeared to be independent of escapement during previous years. Abundance of lake herring in Black Bay varied without trend during the study period despite a strong peak in effective fishing effort during 1968–70 and a general trend of increasing effort throughout the study period. Abundance of Thunder Bay lake herring declined during 1948–50 and 1959–67 and increased when fishing effort declined. The estimated catchability coefficient for small-mesh gill nets (54–83 mm stretched measure) set in Black Bay for lake herring during November was 0.467/102 km net. The estimated catchability coefficient for small mesh gill nets set in Thunder Bay for lake herring during December was 0.132/102 km net. Estimates of natural mortality rates and recruitment levels for both fisheries were confounded and unreliable. This analysis demonstrates the utility of Schnute's model for management of Great Lakes herring stocks and some difficulties with its application.

Impact of Fishing on the Inshore Fishery of Lake Victoria (East Africa)

The inshore fishery of Lake Victoria has symptoms of severe overfishing. Curvilinear multiple regression analysis was used to examine the large variation in fishing effort, gear composition, and catches around the lake and suggest fishing practices that would give highest yields. The overfishing problem is due to the kind of gear in use and not to excessive fishing per se. Heavy fishing is often associated with the use of small mesh gillnets and seines to catch small fish such as Haplochromis, a practice that does not seem to justify the damage done to catches of the larger species by the same gear. The best strategy for maximizing the total tonnage yield is to fish optimally for the herbivorous genus Tilapia. This means using only the larger gillnets appropriate for Tilapia, as well as hooks, both at a very high fishing effort. The hooks capture large predators such as Bagrus, Clarias, and Protopterus, an abundant resource in themselves, and simultaneously appear to increase Tilapia yields indirectly by reducing losses of Tilapia to predators. Key words: Lake Victoria, ecosystem, succession, multispecies fishery, multigear fishery, fish yield, Africa, lake, lungfish, Tilapia, Haplochromis, catfish

The Use and Selectivity of Small-Meshed Gill Nets at Brooks Lake, Alaska

Abstract Fine-thread gill nets with meshes of 3/8-, 1/2-, 3/4-, 7/8-, and 1-inch (stretched measure) were used to sample lake-dwelling juvenile sockeye salmon (Oncorhynchus nerka) and associated small fishes in Brooks Lake, Alaska. Thirteen of 20 species known to occur in the lake were sampled with the small meshes. Sockeye salmon, Alaska blackfish (Dallia pectoralis), threespine stickleback (Gasterosteus aculeatus), ninespine stickleback (Pungitius pungitius) and sculpins (Cottus spp.) less than 50 millimeters in length were sampled. Different selectivity patterns were shown by different species in various mesh sizes. Meshes smaller than 1 inch had low catch efficiencies, although these meshes were useful for qualitative assessment of small fishes.