Increase In Fishing Effort Research Articles

Relationship between elver recruitment and changes in the sex ratio of silver eels Anguilla anguilla L. migrating from Lough Neagh, Northern Ireland

Between 1932–1947 and from 1960 onwards, elvers have been trapped near the mouth of the River Bann, Northern Ireland, and released into Lough Neagh. Each period of elver transport has been followed by a marked increase in the proportion of male silver eels migrating from the lough. Catches of silver eels were sampled on several nights each year from 1965–1974, and the lengths of a total of 20358 eels measured showed a progressive increase in the percentage of male eels from 9.3‐86.0 % during this period. Various reasons for this change were examined. The different ages at which male and female eels migrate to the sea was not important. There was no evidence to support the hypothesis that male elvers normally remain in estuarine conditions, and their transport to the lough was therefore unnatural. An increasing fishing effort for yellow eels, such as occurred following the introduction of trawling in 1960, would favour males since small eels were returned to the lough. It was not thought, however, that this was a major cause of the change in sex ratio. Instead, elver transport appeared to be directly implicated, possibly by the overstocking of Lough Neagh, and the phenotypic determination of progressively more male eels, but the evidence for this suggestion was inconclusive.

Recent changes in the stock of Celtic Sea herring (Clupea harengus L.)

International catches of Celtic Sea herring during the years 1966–70 averaged about 30000 tons/year. This average was considerably above previous estimates for the maximum sustainable yield of this stock. A new assessment of the Celtic Sea herring stock has been made, using information on samples from Dutch catches during the years1958–71. High catches in the years 1966–70 were caused primarily by an increased fishing effort, resulting in an accelerated depletion of older age groups. The increase in catch due to this cause will be mainly temporarily. A second reason for high catches during this period was an increase in level of recruitment by about 50%.

The rainbow trout Salmo gairdneri (Rich.) fishery of Lake Titicaca

Rainbow trout were introduced to Lake Titicaca in 1942. A commercial trout canning operation started in 1961 but terminated in 1970. Data for this study were collected in 1966 and 1967, with the object of examining the life history, biology, and commercial fishery of Titicaca rainbow trout. Trout were caught commercially and experimentally by gillnets in different parts of the lake.Lake Titicaca lies across the border of Peru and Bolivia at 3212 m above sea level. The physico‐chemical characteristics of the lake appear ideal for self‐sustaining populations of rainbow trout. Growth was constant throughout the year, and was good in relation to growth rates of rainbow trout in other parts of the world. Spawning took place in the tributaries in the winter. Recruitment to the fishery occurs in 7.62 cm stretch mesh gillnets at about 27 cm fork length.The total annual commercial catch increased to 500 metric tons in 1965 before subsequently decreasing. The simplest explanation for the decline in total catch would be the increase in fishing mortality due to increased fishing effort, although it is possible that the decline in water level has had some effect. It appeared possible that the lake could sustain an annual catch of about 350 tons.

Effect of Pond Formation on Catches of Brook Trout from a Coastal Stream System

An artificial 5-acre (2.0-ha) pond formed on Ellerslie Brook, Prince Edward Island, initially resulted in an increased fishing effort and yield of brook trout to anglers from the stream system — estuary, pond, and stream. The increased effort and catch were not sustained. After 4 years, annual effort and yield of trout by number from the system were not significantly different from the pre-pond period. Length of angled trout and yield by weight declined significantly. Sea-running trout, prized by anglers, were largely eliminated. Only when planted into the estuary did hatchery-reared trout make good contributions to the anglers' catches.

Electrons provide means of analyzing metal surfaces: Machinery, Vol. 53. No. 5.

Published comparison of data from two sets of cruises in the Adriatic Sea: the first during 1948–1949 and the second during 1996–1998, has shown a significant decrease of demersal fish stocks during 50 years of fishery. Furthermore, it has been shown previously that the stock of Chondrichthyes has decreased while the stock of Osteichthyes has increased relative to Chondrichthyes and, for several species, the increase is in absolute values too.By using Schaefer's model (logistic population under proportional harvesting), the overfished categories are identified. It is also shown that observed changes in Chondrichthyes and Osteichthyes are in agreement with the Volterra principle down to the extinction of predators. Furthermore, by using the Gause competition model under an increasing fishing effort it is shown that an initially dominant competitor may become a marginal competitor, in agreement with observations.Improvements in the management of fish stocks in the Adriatic are proposed. Given the fact that no one so far has determined the optimum harvesting effort of any species set within a marine ecosystem, improvements apply for other seas too.