The idea for an acoustic survey for herring covering the entire North Sea dates back to the early 1950s when Parish (1953) first proposed an "..organised echo search.." to determine the distribution of the species (Fig. 4). However, it was not until 1979, one year after the closure of the North Sea herring fishery, that the first acoustic survey was conducted. Six ships were employed covering the northwestern and west central North Sea and northern part of Division VIa. However, the results of this survey were so variable that the area coverage was reduced to the Orkney/Shetland area the following year (1980).
|(Fig. 4 (reproduction from Parish 1953). Chart showing the proposed echo-search coverage of the North Sea. Dotted line indicates the proposed North Sea echosounder survey grid; solid line the approximate routes followed by some shipping lines and commercial trawl (A = route worked by S/S St Clair; B = approximate route of British Arctic trawlers working from Humber ports).)|
Coverage was restricted to the northern North Sea until 1988 when once again a number of surveys were carried out at sufficiently similar times to enable a combined estimate to be obtained. Since then the ICES international North Sea herring acoustic survey has taken place every year, with the participation of the United Kingdom (Scotland), the Netherlands, Norway, Germany, Denmark and, occasionally, Sweden and the Republic of Ireland.
Typically the survey covers the whole of the North Sea from late June to early August (Fig. 5).The methods used are standardised as much as possible with all vessels using the same frequency (38 kHz) and type of scientific echosounder. The echosounder is calibrated by each vessel on every survey in the same manner and instrument settings are largely the same. Transects are spaced at a maximum of 15 nautical miles and vessels survey speed is 10-12 knots. Most vessels suspend operation at some point at night when the herring become unavailable to the echosounder due to vertical migration into surface waters, although the exact timing depends on the location of the survey.
(Fig. 5. Survey area layouts and dates for all participating vessels in the 2002 international North Sea herring acoustic survey.)