Stock assessment of the pomfret (Pampus argenteus) in Kuwaiti watersMorgan, G. R.
doi: 10.1093/icesjms/42.1.3pmid: N/A
Although it is not yet possible to age the pomfret (Pampus argenteus; locally known as zobaidy) analysis of regular monthly length samples taken during 1982 has enabled estimates of growth, mortality, and mean selection length to be made for the stocks in Kuwaiti waters. The basis of the analysis was the estimation of growth parameters using the ELEFAN I technique of Pauly and David (1981). A length-converted catch curve was then constructed from which total mortality and mean selection length were estimated. Natural mortality (M) was estimated using the relationship between M, the growth parameters, and mean environmental temperature (Pauly, 1980).Yield-per-recruit analysis indicated that the zobaidy stocks are not heavily exploited and an increase in yield per recruit may be expected if fishing effort is increased. However, no significant increase in yield per recruit could be expected from manipulation of size at first capture.Pauly, D. 1980. J. Cons. int. Explor. Mer, 39: 175–192.Pauly, D., and David, N. 1981. Ber. dt. wiss. Kommn Meeresforsch., 28: 205–211.
Growth and feeding of juvenile cod (Gadus morhua L.)Hawkins, A. D.; Soofiani, N. M.; Smith, G. W.
doi: 10.1093/icesjms/42.1.11pmid: N/A
Growth and feeding of immature cod were investigated in Upper Loch Torridon (57°33'N 5°36'W), a fjordic sea loch on the west coast of Scotland. Growth was estimated from mean length and weight at age, and also from the changes in length of tagged fish. Feeding rates were measured by two independent methods: 1) laboratory measurements of the ration levels necessary to produce growth rates similar to those observed in the wild (measured from mean length at age), and 2) examination of the stomach contents of wild fish, by the application of experimentally measured elimination coefficients.Growth rates measured from mean length at age were lower than those measured from tagged fish, perhaps because faster-growing fish leave the loch, biasing mean length at age downwards. There were strong seasonal differences in the growth and condition factor of the cod, both being highest in summer and autumn and lower during winter and spring. The feeding rates estimated by the two independent methods were similar, though those estimated from growth rates were generally lower, probably reflecting an underestimation of growth rate and also differences in the diet and activity levels of laboratory and wild fish. It is suggested that the poor condition and lower food intake of fish in the winter result from a lack of vulnerable prey organisms, rather than any drop in growth efficiency at lower temperatures. The production of juvenile cod in Scottish sea lochs may well be limited by the winter food supply.
The use of parasites as biological tags in population studies of herring (Clupea harengus L.) in the North Sea and to the north and west of ScotlandMacKenzie, K.
doi: 10.1093/icesjms/42.1.33pmid: N/A
The metacercariae of the digeneans Cercaria pythionike Rothschild and Cercaria doricha Rothschild and the plerocerus of the cestode Lacistorhynchus sp. were used to trace recruitment migrations of herring (Clupea harengus L.). The two digeneans have similar life cycles, with the gastropod mollusc Turritella communis Lamarck as first intermediate host, teleost fish as second intermediate hosts, and piscivorous sea birds as definitive hosts. Lacistorhynchus has calanoid copepods as first intermediate hosts, teleost fish as second intermediate hosts, and elasmobranch fish as definitive hosts. All three parasite species occur in the visceral cavities of herring, mainly around the pyloric caeca. Most, if not all, infection of herring with the digeneans occurs in the first year of life but infection with Lacistorhynchus can occur in older herring. All three parasite species have long life spans, probably equal to those of the infected herring.The Northeast Atlantic study area extended from Shetland in the north to the Irish Sea and central North Sea in the south, and from northwest of Ireland in the west to the Kattegat in the east. More than 23 000 herring, aged from 0 to 12 winter rings, and predominantly autumn-spawned, were examined from 1973 to 1982.Examinations of juvenile herring caught on nursery grounds showed that each parasite species infected herring only in certain areas. Identification of the infective area or areas for each species enabled conclusions to be drawn as to the probable origin of different adult herring populations. It was estimated that herring of eastern North Sea-Skagerrak-Kattegat origin in the Minch increased from an estimated 55% of the 2-ring population to 85% of age groups 5 rings and older. Most of the remainder were probably recruited from inshore nursery grounds along the east coast of Scotland and possibly the northeast coast of England. There is evidence that a large proportion of the adult herring caught to the northwest of Ireland were recruited from nursery grounds in the Minch. Adult herring caught in the area to the north and west of the Outer Hebrides had parasite prevalences closest to those in juvenile herring from the eastern North Sea north of 56°N and in the Skagerrak and Kattegat. Parasite prevalences indicated the eastern North Sea-Skagerrak-Kattegat area as the source of recruits to adult herring populations in the northwestern North Sea apart from a component, estimated at 20%, of the 2-ring herring caught in the Orkney—Fair Isle—Clyth Ness area. This component probably recruited from nursery grounds along the east coast of Scotland and possibly the northeast coast of England. There was evidence of an interchange of autumn-spawned herring between populations in the Firth of Clyde and the Irish Sea.
Bio-optical measurements in the Southwest Florida Shelf ecosystemHøjerslev, N. K.
doi: 10.1093/icesjms/42.1.65pmid: N/A
Various daylight and light measurements for optical watermass classification and for in situ photosynthetic studies were made in the Southwest Florida Shelf ecosystem during the period 2–7 April 1982.The water masses in the test area can be subdivided into (i) a surface layer characterized by homogeneity and low concentrations of chlorophyll a and suspended matter; (ii) a thin transition zone where chlorophyll a and suspended matter increase considerably with depth; and finally, (iii) an underlying water layer extending from around the lower limit of the euphotic zone where chlorophyll a and suspended matter gradually decrease. These general features imply rather small horizontal gradients with respect to chlorophyll a and suspended matter when compared with the vertical gradients of the same two parameters. This holds true especially for chlorophyll a. Yellow substance, in terms of the penetration depth of the downward UV-B daylight irradiance, was not measurable in the test area.The colour of the surface water masses, the depth of the euphotic zone, and the Secchi-disc depth varied only slightly at those locations in the test area where these measurements were made and were almost unaffected by the differences observed in the concentration of suspended matter.An algorithm, valid for a large number of both oligotrophic and eutrophic waters, giving the depth of the euphotic zone in terms of the colour of the sea, demonstrated its validity for the water masses in the test area. A similar success was found for another algorithm involving the colour of the sea and the surface concentration of chlorophyll-like plankton pigments.
Factors affecting year-to-year variation in the catch of banana prawns (Penaeus merguiensis) in the Gulf of Carpentaria, AustraliaVance, D. J.; Staples, D. J.; Kerr, J. D.
doi: 10.1093/icesjms/42.1.83pmid: N/A
Annual catch of banana prawns (Penaeus merguiensis de Man) from the Gulf of Carpentaria, Australia, varied six-fold (range 1677–9705 t) over the period 1970 to 1979, although fishing effort remained relatively constant. Relationships between prawn catch and seasonal rainfall, river discharge, air temperature, wind speed and direction, numbers of spawning adults, and numbers of juveniles were determined for six regions of the Gulf using both single variable correlation analyses and multiple regression techniques. Spring, summer, and autumn rainfall were positively correlated with regional catches in the southern regions of the Gulf whereas summer rainfall was inversely related to catch in one northern region. In northern regions, wind and temperature were more closely related to prawn catches than rainfall. However, because the causal mechanisms for temperature and wind effects are unknown, descriptive regression models were developed for all areas using only rainfall variables. These models explained between 35% and 67% of the variation in regional catches. Because rainfall affects the emigration of juvenile prawns, estimates of the abundance of earlier life history stages were of little value in explaining subsequent catches. Predictive models based on rainfall data could provide estimates of catches in the southern regions of the Gulf up to six weeks prior to the prawn season each year. In northern regions, where annual variations in catches are less than those in southern regions, the most reliable estimate at this stage of data collection is the mean catch for the past seven years in each region. No long-term trends in prawn catches over the last decade have occurred that cannot be explained by variations in environmental variables over the same period, although recruitment overfishing cannot be completely ruled out.