Fisheries have rarely been 'sustainable'. Rather, fishing has induced serial depletions, long masked by improved technology, geographic expansion and exploitation of previously spurned species lower in the food web. With global catches declining since the late 1980s, continuation of present trends will lead to supply shortfall, for which aquaculture cannot be expected to compensate, and may well exacerbate. Reducing fishing capacity to appropriate levels will require strong reductions of subsidies. Zoning the oceans into unfished marine reserves and areas with limited levels of fishing effort would allow sustainable fisheries, based on resources embedded in functional, diverse ecosystems.

/pdf/towards-sustainability-in-world-fisheries-26u29h5wj5.pdf

Towards sustainability in world fisheries

Quantitative fisheries stock assessment : , Quantitative fisheries stock assessment : , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Quantitative fisheries stock assessment

Numerous attempts have been made to estimate the production in the sea of fish and other organisms of existing or potential food value to man (1-4). These exercises, for the most part, are based on estimates of primary (photosynthetic) organic production rates in the ocean (5) and various assumed trophic-dynamic relationships between the photosynthetic producers and the organisms of interest to man. Included in the latter are the number of steps or links in the food chains and the efficiency of conversion of organic matter from each trophic level or link in the food chain to the next. Different estimates result from different choices in the number of trophic levels and in the efficiencies, as illustrated in Table 1 (2). Implicit in the above approach is the concept of the ocean as a single ecosystem in which the same food chains involving the same number of links and efficiencies apply throughout. However, the rate of primary production is known to be highly variable, differing by at least two full orders of magnitude from the richest to the most impoverished regions. This in itself would be expected to result in a highly irregular pattern of food production. In addition, the ecological conditions which determine the trophic dynamics of marine food chains also vary widely and in direct relationship to the absolute level of primary organic production. As is shown below, the two sets of variables primary production and the associated food chain dynamics may act additively to produce differences in fish production which are far more pronounced and dramatic than the observed variability of the individual causative factors.

https://science.sciencemag.org/content/sci/166/3901/72.full.pdf

Photosynthesis and Fish Production in the Sea

Fish lengths have been measured in many different ways The differences arise from choosing different reference points near the anterior end and near the posterior end of the fish, and from using different methods of making the measurement Methods of making the measurement include using calipers; using a tape held along the curve of the body; laying the fish on a measuring board with the front end pressed against an upright piece; laying the fish on a board with a movable cross hair above it, attached to an indicator running along a scale In theory, any combination of reference points and methods might be used, but practice is considerably more restricted Some of the commoner combinations have special names; these are provided in the chapter, together with the reference points used In addition to these methodological differences, length varies with the condition of the fish—for example, whether it is alive, recently killed, after rigor mortis has set in, or at different intervals of time after preservation in formalin or alcohol

11 – Growth Rates and Models

Density-dependent processes such as growth, survival, reproduction and movement are compensatory if their rates change in response to variation in population density (or numbers) such that they result in a slowed population growth rate at high densities and promote a numerical increase of the population at low densities. Compensatory density dependence is important to fisheries management because it operates to offset the losses of individuals. While the concept of compensation is straightforward, it remains one of the most controversial issues in population dynamics. The difficulties arise when going from general concepts to specific populations. Compensation is usually quantified using some combination of spawner–recruit analysis, long-term field monitoring or manipulative studies, and computer modelling. Problems arise because there are limitations to each of these approaches, and these limitations generally originate from the high uncertainty associated with field measurements. We offer a hierarchical approach to predicting and understanding compensation that ranges from the very general, using basic life-history theory, to the highly site-specific, using detailed population models. We analyse a spawner–recruit database to test the predictions about compensation and compensatory reserve that derive from a three-endpoint life-history framework designed for fish. We then summarise field examples of density dependence in specific processes. Selected long-term field monitoring studies, manipulative studies and computer modelling examples are then highlighted that illustrate how density-dependent processes led to compensatory responses at the population level. Some theoretical and empirical advances that offer hope for progress in the future on the compensation issue are discussed. We advocate an approach to compensation that involves process-level understanding of the underlying mechanisms, life-history theory, careful analysis of field data, and matrix and individual-based modelling. There will always be debate if the quantification of compensation does not include some degree of understanding of the underlying mechanisms.

/pdf/compensatory-density-dependence-in-fish-populations-1bounl0hxh.pdf

Compensatory density dependence in fish populations: importance, controversy, understanding and prognosis

Some aspects of the dynamics of populations important to the management of the commercial marine fisheries

Postwar growth of world marine fisheries has been rapid, from 17 million metric tons in 1948 to 40 million in 1962, and recently seems to have accelerated. Crude estimates of the potential total harvest may be made on the basis of information about unfished regions and underexploited stocks. These lead the author to conclude that, without radical changes in fishing techniques, or radical intervention, as for example by extensive aquaculture, the harvest can easily be increased by a factor of two, and probably four. It should also be possible to estimate potential harvest from net photosynthetic productivity and rates of transfer of materials through the food web to harvestable species, but such estimates are imprecise due to lack of good information about routes and rates of transfer. However, they support a conclusion that a potential harvest of 200 million tons a year is probably conservative. By harvesting species lower in the food chain, in areas where they occur in economically catchable agg...

The Potential Harvest of the Sea

ENGLISH: Strong coastal upwelling occurs in the Gulf of Panama regularly each year during the season, from about January through April, when strong northerly winds are blowing offshore. Because of the evident importance of upwelling to the ecology of the Gulf of Panama, we commenced in the fall of 1954 a study of various physical, chemical, and biological phenomena associated therewith. Observations were taken at bi-weekly intervals at a fixed location in the Gulf (approximately 10 miles SE of Taboga Island) to supplement the serial observations of sea level, sea temperature, and winds that have been gathered for many years by the Panama Canal Company. SPANISH: Cado ano, en la estacion de enero a abril, cuando los vientos del norte
soplan vigorosamente frente a la costa, ocurre en el Golfo de Panama un fuerte afloramiento costanero. Se cree que este afloramiento periodico en el Golfo de Panama es responsable de la alta productividad biologica que sostiene considerables cantidades de organismos de importancia comercial. Esta region, por ejemplo, es una fuente importante de la especie Cetengraulis mysticetus) pez de carnada para el atun, (Alverson y Shimada, 1957) y mantiene una considerable pesca de camarones llamados langostinos (Burkenroad, Obarrio y Mendoza,1955).

(PDF contains 54 pages.)

Some aspects of upwelling in the Gulf of Panama

Attachment of the Larvae of Ostrea Gigas, the Japanese Oyster, to Plane Surfaces

The yield from an exploited fish population depends on the rate of harvesting (fishing mortality rate) and the magnitude of the standing stock. The latter is determined by rates of increase from recruitment and growth and rates of loss from both natural and fishing mortality. Since we lack information respecting the density-dependence of each of these rates, various simplifying assumptions are made in practice in developing mathematical models of exploited fish populations. Such models are described, with illustrations of their application to important commercial fisheries. Models of fisheries involving competing fish species, and the employment of computer simulation in studies of fishery-dynamics are also discussed.

Milner B. Schaefer

Papers

Some aspects of the dynamics of populations important to the management of the commercial marine fisheries

The Potential Harvest of the Sea

Some aspects of upwelling in the Gulf of Panama

Attachment of the Larvae of Ostrea Gigas, the Japanese Oyster, to Plane Surfaces

Methods of Estimating Effects of Fishing on Fish Populations