Spawning-stock biomass of scad (Trachurus trachurus L.) to the west of the British Isles, as indicated by egg surveysEaton, D. R.
doi: 10.1093/icesjms/45.3.231pmid: N/A
The distribution and abundance of stage I eggs of scad (horse mackerel,Trachurus trachurus L.) obtained during joint international plankton surveys on the European continental shelf to the west of the British Isles in 1977, 1980, 1983, and 1986 are described, and estimates of stage I scad egg production during the surveys are given. These results have been used to provide the first estimates, independent of catch and age data, of the spawning-stock biomass (SSB) of scad in the survey area.Data from the Biscay area (south of 48°N, ICES Subarea VIII) and the Celtic Sea/West of Ireland area (48°N to 54°N, ICES Subarea VII) have been analysed separately; the indications are that whilst the SSB has remained relatively stable in Biscay over the period covered by the surveys, there has been a decline in areas north of latitude 48°N. The SSB estimates are discussed in the light of the problems which exist in determining the real, as opposed to potential, fecundity of scad.
The distribution of transferrin alleles in haddock stocksJamieson, A.; Birley, A. J.
doi: 10.1093/icesjms/45.3.248pmid: N/A
This paper concerns the use of transferrin alleles for discriminating between natural populations of haddock. Electrophoretic analyses of blood plasma in haddock (Melanogrammus aeglefinus L.) have revealed qualitative variations in transferrin molecules. The variations are attributed to a series of 21 co-dominant alleles at the Tf gene locus in haddock and were used in a study of haddock in the vicinity of the British Isles between 1976 and 1983. Haddock were sampled extensively over the northern North Sea and at Faroe, the Hebrides, and Rockall. The material contained fifteen consecutive year broods which were spawned from 1967 to 1981. Sexes were determined. Blood specimens were tested for isoallelic forms of transferrinStatistical analyses of the geographical distribution of transferrin alleles indicated two races of haddock in the North Sea, and at least three, and possibly four, distinct races of haddock in the surveyed areas. A genetically continuous race of haddock in Scottish waters extends around the Hebrides, the Northern Islands, and the east coast. A different race of haddock occurs in the eastern longitudes of the North Sea. The latter race is almost indistinguishable from the haddock at Faroe, but the Faroe haddock population contains a higher incidence of rare alleles. Rockall haddock is another distinctive race.
The effect of different sweep lengths on the length composition of bottom-sampling trawl catchesEngås, Arill; Godø, Olav Rune
doi: 10.1093/icesjms/45.3.263pmid: N/A
Abundance estimates of Northeast Arctic cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L) are compiled annually from combined bottom-trawl and acoustic surveys. Both survey methods use data from bottom-trawl catches in the calculations. The surveys are conducted in the Barents Sea and the Svalbard area with the same bottom trawl, but with different sweep lengths. The bottom-trawl indices are calculated for both areas assuming an effective path width of 25 m.In this paper results from parallel trawling experiments with the standard sampling trawl equipped with different sweep lengths are analysed. The results show that the total catch increases with increasing sweep length. Small fish are relatively underestimated by the trawl with the longer sweeps, and a slight species selection was indicated. It is concluded that combining survey results without compensating for the effect of using different sweep lengths will bias the estimates.
Escape of fish under the fishing line of a Norwegian sampling trawl and its influence on survey resultsEngås, Arill; Godø, Olav Rune
doi: 10.1093/icesjms/45.3.269pmid: N/A
The Institute of Marine Research, Bergen, has carried out combined bottom-trawl and acoustic surveys for cod and haddock in the Barents Sea and the Svalbard area since 1981. The results have demonstrated clearly that the current trawl and acoustic survey methods considerably underestimate the young age groups of cod, partly because of low gear efficiency for the smaller fish.This paper describes the results of experiments with small bags to collect fish that escape under the trawl. Catches of several different species were obtained in the small bags. Comparison between trawl and bag catches reveals a length-dependent escape of cod and haddock under the trawl; i.e., small fish are severely underrepresented in the trawl catches. The escape also has a considerable effect on the species composition, because cod, more than haddock, tend to dive under the fishing line. To improve the efficiency of the sampling trawl, a rockhopper ground gear was introduced. Comparison of the trawl fitted with standard bobbins gear and with rockhopper gear showed that escape under the standard equipment was of similar magnitude as that in the bag experiment.
The diurnal rhythm in spawning of plaice (Pleuronectes platessa L.) in the southern North SeaNichols, J. H.
doi: 10.1093/icesjms/45.3.277pmid: N/A
The diurnal rhythm in spawning of plaice (Pleuronectes platessa L.) in the southern North Sea was investigated using two series of plankton samples taken over 24-h periods in 1982 and 1984. In order to calculate the fertilization times of recently spawned eggs in these samples, new data on the rate of development of the blastodisc stage were obtained experimentally. Standard survey data and replicate hauls in the centre of spawning in 1982, 1983, and 1984 were also analysed.All of the samples examined, including a re-analysis of Simpson's (1971) data set, confirm that approximately90 % of spawning in this species occurs at night.The time-series data available from these surveys were insufficient to test whether spawning showed a lunar periodicity.
North Sea benthos: A review of field investigations into the biological effects of man's activitiesRees, H. L.; Eleftheriou, A.
doi: 10.1093/icesjms/45.3.284pmid: N/A
This paper reviews English-language publications on the benthic fauna of the North Sea, with particular reference to those studies aimed at assessing the effects of man's activities. Coverage includes estuaries and coastal waters of the eastern United Kingdom, Belgium, The Netherlands, the Federal Republic of Germany, and the western coastlines of Denmark, Sweden, and Norway, as well as offshore areas.There are regional differences in the amount of evidence available for linking changes in the coastal benthos with anthropogenic influences, reflecting not only differences in the nature and magnitude of activities, but also variations in coastal topography and prevailing hydrographic conditions which may or may not ameliorate the effects.Where such effects have been clearly demonstrated, these have invariably been confined to areas of limited water exchange, or in the vicinity of discharge points. Offshore, the weight of evidence favours the view that no significant benthic changes appear to occur as a result of waste discharge, excepting close to known sources of input, e.g, oil production platforms. However, these changes are very localized and are of little significance relative to the sea area as a whole.Recently, attention has been focused on the possible consequences for the marine ecosystem of an increasing trend of nutrient inputs to certain coastal areas from anthropogenic sources. Regarding the benthos, some comparisons with historical data have been possible, though as yet there is no unequivocal evidence to link changes - where observed - with eutrophication.A persistent concern in benthic studies is the ability to account for, and hence distinguish between, natural effects and those which may be attributable to low-level contamination. This is especially true in inshore environments where marked seasonal or year-to-year changes in abundance and biomass of short-lived species are commonly recorded. In the longer term, benthic communities may also respond to cyclical changes in climate on a variety of time scales. These factors emphasize the general value of extended time-series data in the evaluation of ecosystem changes, and also the importance of “control” sites in monitoring trends in the biota in relation to contaminant inputs.