Biology

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Whiting (Merlangius merlangus) is one of the smaller gadoid fish species that is of importance to Scottish Fishermen. There are two main stocks, the North Sea and west coast, that contribute to Scottish landings of 9,200 tonnes worth £9.3 million. A variety of types of trawl net are used to catch whiting and they occur in a number of mixed fisheries.

Whiting is one of the most numerous and widespread species found in the North Sea. High numbers of immature fish occur off the Scottish coast, in the German Bight and along the coast of the Netherlands. Tagging experiments, and the use of a number of fish parasites as markers, show that the whiting found to the north and south of the Dogger Bank form two virtually separate populations. It is also likely that the whiting in the northern North Sea may contain populations including 'inshore' and 'offshore' groups. A separate population of whiting is widely distributed throughout the west coast of Scotland with immature fish occurring in sea lochs and inshore areas and older fish moving offshore. Whiting found to the south of 56°N and to the west of Ireland are separate from those in the Minches, the Clyde and the Irish Sea. The whiting population that over-winters in the Stanton Bank area moves to the Irish coast and the Clyde to spawn in spring. Some individuals may even move to the west of the Hebrides.

At four years old, a single female fish of reasonable size produces more than 400,000 eggs. By two years old however, most whiting are mature and able to spawn. The spawning season lasts from late January until June. The spawning season of an individual female lasts 10-14 weeks, during which time she releases many batches of eggs. Like many other fish, whiting spend their first few months of life in the upper water layers before moving to the seabed. They grow very quickly for the first year, after which the growth rate becomes much slower. There are large differences between the growth rates of individual fish and a 30 cm fish can be as young as one year or as old as six. The whiting in the northern North Sea usually grow faster than their more southern counterparts.

Adult whiting feed mainly on juvenile fish and crustaceans (shrimps and crabs). The exact composition of the diet depends on the size of the fish, the area and the time of the year. In the North Sea, whiting is one of the main predators of other commercially important species of fish. Norway pout, sandeel, haddock, cod and even whiting themselves are frequently eaten. It has been estimated that each year, depending on the size of the whiting population, consumption of these species is of the order of a hundred thousand tonnes.

whiting_maps
Spawning Grounds

In the absence of defined reference points, the state of the North Sea stock cannot be evaluated. An analytical assessment estimates Spawning Stock Biomass in 2009 as being at the lowest level since the beginning of the time-series in 1990. Fishing mortality has declined from 2000-2004, but increased in recent years. Recruitment has been very low since 2002, with an indication of a modest improvement in the 2007 year class.

No new assessment for the west coast stock was carried out in 2009 for the 2010 fishery. The most recent landings and survey data available for this stock did not change the perception of the stock and do not give any reason to change the advice from 2007 or 2008. Therefore, the advice for the 2010 fishery is the same as the advice given for the 2008 and 2009 fishery.

The following information was provided in the 2007 advice and has not changed since then:

"The state of the [West coast] stock is unknown, but long-term information on the historical yield and catch composition and survey-based assessment covering the more recent period all indicate that the present stock size is at a historical low. Total mortality has been higher in the last decade than in the previous one. Recruitment in the most recent years is estimated to be very low".

North Sea

Because there are no reference points for the North Sea whiting stock, ICES advises on the basis of precautionary considerations that a significant reduction of the TAC is required to remedy the decline in SSB. An immediate TAC reduction of 61% (13,400 t Total catch, 7,400 t human consumption catch) is needed to stabilise the stock, but rebuilding would require a further reduction.

Catches of whiting have been declining since 1980 (from 224,000 t in 1980 to 27,000 t in 2008, including discards and industrial bycatch). Distribution maps of survey IBTS indices show a change in distribution of the stock which is now located mainly in the northwestern North Sea. Catch rates from localised fleets may not represent trends in the overall North Sea and English Channel population. The localised distribution of the population is known to be resulting in substantial differences in the quota uptake rate. This is likely to result in localised discarding problems that should be monitored carefully.
The basis of the advice has changed from last years, which set a maximum fishing mortality rate (Fmax). This year's advice is aimed at stabilising the SSB.

The perception of the whiting stock to the west of Scotland has not changed since a survey based assessment was carried out in 2007. This was considered to be a reasonable indicator to evaluate long-term trends in the spawning stock biomass, total mortality and recruitment. Survey information indicated an increase in unaccounted removals from this stock, thus biasing absolute biomass estimates from landings data, and preventing an accurate catch-at-age assessment being used for management purposes. The decrease in survey biomass in recent years implies that unaccounted removals took place which caused some harm to the stock.
With the increase in minimum mesh size from 100 mm to 120 mm in 2001/2002 and with the introduction of effort regulations, there has been a shift to smaller meshed gears to target the valuable Nephrops and gain more days at sea. This has resulted in poorer selectivity of fish species leading to an increase in discarding and high grading.
Given that the spawning stock biomass is estimated at the lowest observed level and total mortality at the highest level over the time period, ICES recommends that catches in 2010 be reduced to the lowest possible level.

	EU TAC	UK share
North Sea IV	11,044	7,391
West of Scotland VI	431	246

None at present