Technical methods

Technical methods (Sheperd, 1993; Coffey, 1995) aim to ensure that few young fish are caught before they mature. Methods therefore include:

(a) minimum mesh sizes of nets and sizes of hooks;

(b) minimum landing sizes of fish;

(c) closed areas, e.g. nursery areas;

(d) spawning season closures.

Minimum mesh sizes

It was at one time argued that mesh regulations for trawlers must prove ineffective in protecting young fish because it was thought that, during trawling, the net becomes pulled out lengthways, almost completely closing the meshes so that any small fish entering the cod end must be retained. This was disproved in experiments by Davis and Goodchild (Davis, 1934). They enclosed the cod end of a trawl with a long bag of fine-mesh netting capable of trapping any small fish that might escape through the cod end. The fine-mesh bag was encircled by a noose which after a period of trawling was automatically drawn tight, closing the bag. This demonstrated conclusively that large numbers of small fish passed through the cod end unharmed.

Placing a fine-mesh bag around the cod end also provides a means of studying the extent to which different sizes of fish are retained by different meshes of trawl. By counting the number of each size of fish in the cod end and in the bag, these data can be used to construct a graph showing the percentage of each size of fish that escape through the mesh. This is termed a percentage release curve or selection ogive, and usually has the form shown in Figure 9.36. We see that, for each mesh size, all fish above a certain size are retained in the net. There is also a lower limit of size beneath which virtually all the fish escape. Between these two limits, the proportion of fish which are caught or escape varies with their length, there being a particular size at which 50 per cent of the fish escape. This 50 per cent release length is a convenient index for the comparison of the selective action of different meshes.

There is, of course, no size of mesh that is equally suitable for all species of fish. If the mesh is selected primarily to protect young cod, it will let an undue proportion of haddock escape. A mesh suitable for plaice will let through most of the sole. In the case of large fish such as cod, total reliance on mesh size

10 20 30 AO

Length of fish (cm)

Figure 9.36 Selection ogive for plaice in nets of 80 and 140 mm.

(From Wlmpenny 1953, by courtesy of The Buckland Foundation.)

10 20 30 AO

Length of fish (cm)

Figure 9.36 Selection ogive for plaice in nets of 80 and 140 mm.

(From Wlmpenny 1953, by courtesy of The Buckland Foundation.)

restrictions for conservation of the stock would necessitate a mesh size of about 200 mm which is much larger than that currently in use (currently 100 mm). Estimates of how the spawning stock biomass of North Sea cod would increase or decrease with changes in mesh size used and in relation to fishing intensity have been calculated (Shepherd 1993). Fisheries biologists widely consider 20 per cent of the unfished level to be the minimum desirable level for spawning stock biomass. Another lower level, the minimum tolerable level, is also calculated. Below this level, biologists consider the stock does not have a better-than-even chance of maintaining its size through recruitment. At current fishing levels, with the current mesh size of 100 mm, the stock would be kept below not only the minimum desirable level but also below the minimum tolerable level. To alleviate this by changing mesh size alone, without restrictions on catch and effort, would require an increase in mesh to at least 140 mm and preferably 180 mm. This is not practical. In the Georges Bank fishery for cod, haddock and yellow flounder, all large species, reliance on a minimum mesh size of 140 mm for the past 10 years has led to a drastic decline (Sheperd, 1993).

Direct methods

Direct methods aim to limit the quantity of the catch and to make sure the total death rate is low enough to ensure stock maintenance even if young fish are caught. Methods include:

(a) limits on catches, e.g. total allowable catch per annum (TAC);

(b) limits on the length of the fishing season or hours of fishing permitted;

(c) limiting the size of the fishing fleet;

(d) temporary bans on the fishing of particular over-exploited species in particular areas.

Thus, although the suitability of different methods of control varies with the nature of the fishery, it is generally necessary to set a limit to the total catch of a particular species, as other protective measures may not be completely effective in preventing overfishing because the fishing effort can often be sufficiently increased to circumvent them.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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