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Fish stocks are population of a particular species of fish, for which intrinsic parameters (growth, recruitment, mortality and fishing mortality) are traditionally regarded as the significant factors determining the stock's population dynamics, while extrinsic factors (immigration and emigration) are traditionally ignored. Stocks fished within biologically sustainable levels decreased from 90% in 1974 to 62.3% in 2021.
For fish, it is rare for an individual to reproduce randomly with all other individuals of that species within its biological range. There is a tendency to form a structured series of discrete populations which have a degree of reproductive isolation from each other in space, in time, or in both. This isolation is reflected in the development between sub-populations of differences, morphological variations and exposure to different chemical regimes and parasite species. Sub-populations also respond to fishing in such a way that fishing on one population appears to have no effect on the population dynamics of a neighbouring population.
The currently accepted definition of a stock in fisheries science, is that of Begg et al. (1999), “…a describes characteristics of semi-discrete groups of fish with some definable attributes which are of interest to fishery managers.” Stock identification is a field of fisheries science which aims to identify these subpopulations, based on a number of techniques.This concept is applied in sustainable development goal 14 where target 14.4 is measured a fish stock sustainability indicator 14.4.1.
Straddling stocks are usually Pelagic fish, rather than Demersal fish. Demersal species move less than pelagic species, since they tend to relate to bottom topography. Pelagic species are more mobile, their movements influenced by ocean temperatures and the availability of zooplankton as food. Example pelagic fish are capelin, herring, whiting, mackerel and redfish, There are, however, a few demersal species that are straddling, such as the Greenland halibut migrates in feeding/spawning migrations to Greenland in the west and to the Faeroes in the east. Pelagic and straddling stocks
Straddling stock can be compared with transboundary stock. Straddling stock range both within an EEZ as well as in the high seas. Transboundary stock range in the EEZs of at least two countries. A stock can be both transboundary and straddling.FAO (2007) ftp://ftp.fao.org/docrep/fao/010/i0150e/i0150e00.pdf Rome, Fisheries Report No. 829.
In particular, to ensure continued, healthy, fish stocks, measurements of the Spawning Stock Biomass (the stock population capable of reproducing) allows sensible conservation strategies to be developed and maintained through the application of sustainable fishing quotas. International Council for the Exploration of the Sea (ICES) website
The World Wildlife Fund and the Zoological Society of London jointly issued their "Living Blue Planet Report" on 16 September 2015 which states that there was a dramatic fall of 74% in world-wide stocks of the important scombridae fish such as mackerel, tuna and between 1970 and 2010, and the global overall "population sizes of mammals, birds, reptiles, amphibians and fish fell by half on average in just 40 years." Living Blue Planet Report wwf.org.au September 2015 It is hoped that global efforts like the United Nations Sustainable Development Goal 14 meets its targets aimed at reversing these trends.
The UN Food and Agriculture Organization found that stocks fished within biologically sustainable levels decreased from 90% in 1974 to 62.3% in 2021..
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