Small-mesh Nets Research Articles

Effects of fishing on the Tilapia nilotica (Linne‘ 1757) population in Lake George, Uganda over the past 20 years

SummaryThe size of the Tilapia nilotica population in Lake George was determined using purse seines. Production, mortality rate, size at maturity, and other parameters of the population were also determined. In Lake George, the factors that regulate the population of T. nilotica are both natural and artificial. The restricted size of nursery zones, strong fish‐killing storms, predation and old age are some of the natural regulators. For the adult population, mortality due to these factors is low (natural mortality coefficient, M=0.1). However, commercial fishing appears to have a stronger controlling force. Since the intensification of commercial fishing in 1950, the size of the fish caught has been decreasing, whereas the number of fish caught has been increasing yearly, probably as a result of increasing effort and the use of small mesh nets. These two trends may be the cause for T. nilotica to mature at a smaller size than they did 15 years ago.

稚魚網による炉水量および採集量の変動について

The present study's objective has been to clarify the tow to tow variations in the number of fish larvae taken in relation to the variation of the volume of water filtered. More than 230 surface towings and 170 towings of subsurface layers were carried out in the regions off central and southern Japan, and the results obtained are as follows: (1) Volume of water filtered vary from tow to tow, net to net and it is difficult to keep the volume of water filtered to a constant level by only maintaining a constant ship speed and towing time. (2) Collecting efficiency both in number and size appeared better in small meshed nets than large meshed nets at sea areas of poor plankton. However, the difference in the numbers collected diminished at towings in rich plankton waters, and the variation in collection became large with small meshed nets. (3) Variations in collection was large for samples of low collection and this decreased by increasing the amount of collection in numbers. (4) Movement of net during towing was stable at shallow layers and at calm seas and it oscillated to some extent at deeper layers and seas having complexed water stratification.

The fallow deer situation in Epping Forest, Essex, England

Selection curves of small mesh trawls were assessed with one alternate haul and two trouser trawl experiments. The effect of fish length on the ratio (pi) of the number of fish of length i caught in the large mesh cod-end to the number of fish of length i caught in both cod-ends was estimated with generalized linear modeling. The alternate haul experiment was conducted with beam trawls equipped with 2 and 18 mm stretched mesh cod-ends to assess size selectivity for perch Perca fluviatilis. None of the perch that entered the cod-end could escape through the mesh of the small mesh cod-end. For small perch (<ca. 10 cm total length), pi reflected the ratio in areas swept by the large and small mesh nets, but for larger perch, pi increased. Water flow through the small mesh net must have been lower than through the large mesh net, enabling relatively more larger perch (>ca. 10 cm total length) to out-swim the small mesh net. The trouser trawl experiments were conducted to study selectivity for smelt Osmerus eperlanus, using 12 and 22 mm stretched mesh cod-ends, and for American plaice Hippoglossoides platessoides, using 39 and 155 mm stretched mesh cod-ends (published data). During both trouser trawl experiments, smaller fish that entered the cod-end could escape through the large mesh cod-ends, so pi increased with increasing fish size. However, also beyond the size where fish could escape through the large mesh cod-end, pi continued to increase. This implied that the small mesh cod-end was less selective to larger fish. We demonstrated that the lower selectivity to larger fish in the cod-end with the smaller mesh could result in 20–50% overestimation of the length (L50) at which 50% of the fish entering the cod-end escape through the mesh.

Studies of Haddock in the Passamaquoddy Bay Region

From 1954 to 1957 haddock within Passamaquoddy Bay have been of intermediate sizes. Few small fish or large fish have been captured either by commercial fishermen or in small-mesh research nets. Tagged fish of the Passamaquoddy Bay region moved out of the Bay during winter and mingled mainly with haddock stocks off the New England States. In the following summer recaptures of tagged fish were again most numerous within Passamaquoddy Bay. Stocks of haddock within the Bay appear to result from annual migrations into the Bay in early summer. It is predicted that the proposed power structures would have no effect on haddock stocks outside the high and low pools but that they would probably seriously reduce the haddock population within the high pool.

The near Extinction of Lake Trout in Lake Michigan

After the collapse of the commercial fishery for lake trout (Salvelinus namaycush) in Lake Michigan in the late 1940ˈs, the further decline of the population was traced by records of numbers of small lake trout (mostly 11 to 16 inches in total length and 3 to 5 years old) caught in small-mesh nets of the chub (Coregonus \[= Leucichthys\] spp.) fishery. By 1951 the estimated abundance of these lake trout in lower Lake Michigan was only about 4 percent of their abundance prior to the invasion of the sea lamprey (Petromyzon marinus). This remnant of the population declined severely in subsequent years to a point near extinction by 1955. In April-July 1955 only 8 lake trout were caught in 5 1/2 million linear feet of gill net. Statistics of the lake trout and chub fisheries in State of Michigan waters of Lake Michigan for the years 1929–1954 give no indication that the destruction of small lake trout in the chub fishery had any effect on the later abundance of lake trout of commercial size, or that this destruction was a significant factor in the collapse of the lake trout fishery. Lake trout were brought near extinction by lethal attacks of the sea lamprey and by the near or perhaps complete failure of natural reproduction in 1949 and subsequently. Comparisons in 1949 and 1950 of numbers of legal-sized lake trout caught in large-mesh nets with numbers of small fish taken in chub nets showed that both large and small lake trout declined over the same period, and that by these years the decline may have been greater among small than among legal-sized fish.

The Pelagic Young and Early Bottom Stages of Teleosteans

The present report deals with the young fish which were collected in the neighbourhood of Plymouth during the summer and autumn of 1919, from the beginning of July to the end of September, by small meshed nets operated from the Association's steamer Oithona. It is a supplement to Dr. Allen's report on the “Post-larval Teleosteans collected near Plymouth during the Summer of 1914,” published in Vol. XI, No. 2 (May, 1917), of this Journal, and to the writer's “General Report on the Larval and Post-larval Teleosteans in Plymouth Waters” in Vol. X, No. 2 (June, 1914).The total number of fish examined amounts to 16,400, which are referable to 24 families, 39 genera and 71 species. All the forms are stages in the life histories of fishes, which are well known as adults.The pelagic young of the species of Gadus and Pleuronectes are poorly represented in the catches, but this is not surprising when one considers the lateness in the year when the hauls were made. Similarly the young of early spawners like the Herring are absent.The pelagic young of the Hake (Merluccius merluccius) appear for the first time since investigations with small meshed nets have been undertaken in the Plymouth area, the only other records of their occurrence in the western area of the English Channel being those which were taken by the Danish research steamer Thor, vide Schmidt (1909).

The Post-Larval Stages of Ammodytes Species Captured during the Cruises of s.s. “Oithona” in Plymouth Waters in the Year 1919

The post-larval stages of Ammodytes species captured in the young fish trawl and small-meshed nets during the cruises of s.s. Oithona in the year 1919 were separable into two well-defined series, A and B respectively. Details of the hauls of the trawl and nets with dates and localities are to be found on pages 210–212 of Mr. R. S. Clark's account of the cruises in the present volume.Considering the adult forms of Ammodytes known to occur in Plymouth waters, it is observed that Ammodytes lanceolatus, Lesauvage, and Ammodytes tobianus, L. are frequently met with, but no record is to hand at present of the occurrence of Ammodytes cicerellus, Rafin., although the latter has been taken frequently in the Atlantic. In regard to the larvæ and post-larvæ, none comparable to those assigned to A. cicerellus by Louis Fage (1, p. 17) have been described previously as occurring in this district, nor have such forms revealed themselves in the material under present consideration.In endeavouring to identify the post-larvæ of any species, a study of the characters of the adults is often of great help, but in the case of Ammodytes species the only features of the adult which seem to be of value in this respect are the number of vertebræ and the relative length of the head to the total length of the body, and even so these characters, on account of the difficulty of determining them, are not so helpful as they would appear to be. Other characters have therefore to be sought for, and the following are those by which Series A and B have been distinguished:—