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doi: 10.1006/jmsc.1993.1013pmid: N/A
There are many examples in the literature of correlations between changes in physical oceanographic factors and changes in fish stocks. Commonly, the correlations hold for a few years, then break down. This does not necessarily mean that a correlation was invalid, but calls for a better understanding of the mechanism of interaction in all its complexity before we can understand correlations that change with time. In this review attention is focused on biological production processes as links between the physical phenomena and the fish stocks. There are two fairly distinct food chains in the plankton, one based on diatoms and the other based on bacteria and flagellates. Diatoms are consumed by mesozooplankton or by benthos (if they sink), and these organisms form the food of most commercially important fish stocks. Bacteria and flagellates are consumed by microzooplankton and enter a complex food web that is inefficient in supporting fish production. There is a pattern of events in the water column that favours diatom production, and hence fish production. It consists of a period of vertical mixing followed by a period of stratification. It was first described in connection with the spring bloom in temperate waters, but it is also found, for example, in estuaries, coastal upwelling systems, and tidal fronts. The most important forces causing vertical mixing are wind and tidal currents. Variations in wind strength from year to year provide plausible explanations for variations in fish productivity. Stratification is the result of either solar heating or freshwater input. Many changes in fish stocks can be seen to result from changing river runoff or influx of a layer of warm water. In recent years evidence has been accumulating for synchronous changes in widely separated stocks. Sardines in the Californian and the Peruvian upwelling systems have varied synchronously with the Far Eastern stocks. Three species of salmon in the north Pacific have varied synchronously over the last 60–70 years. Periods of El Niño, when Peruvian sardine catches were low, corresponded to periods of good catches of jack mackerel off Tasmania. In many cases, the mechanisms of such correlations are poorly understood. Yet it seems almost certain that global patterns of atmosphere–ocean interaction are in some way responsible. Stocks have varied synchronously in spite of widely different management patterns, and it looks as if physical factors, operating through marine food webs, are often the dominant forces for change in the fish stocks.
doi: 10.1006/jmsc.1993.1014pmid: N/A
Back-calculation formulae (BCF) frequently used for deriving fish growth can only be applied when considering a predefined reference axis to measure growth rings and daily growth increments on otoliths. Many otoliths are difficult to interpret and it is often not possible to age fish along the predefined axis. A back-calculation model which avoids this constraint of standardization is presented. This method is based on the elliptical modelling of a quadrant of the otolith's transverse section plane, using data recorded with an image analysis system. Considering the nucleus as the ellipse center, the two parameters “a” and “b”, the large and small axes of the ellipse, respectively, are estimated. When the longitudinal axis of the section and the otolith nucleus are taken as references, a point from the otolith edge or from one ring can be defined by its polar co-ordinates: α = angle between the interpreted axis and the longitudinal axis, r = distance from the nucleus to the intersection between the interpreted axis and the margin or a particular zone of reading (annulus or daily growth increments zone). We use these co-ordinates and replace a and b by their expression as a function of the fish length (L) in the ellipse analytical formulation. The BCF obtained is therefore of the type L = f(L, r, α), (f is a function describing the relationship between L, r, α) and affords, knowing that r and α characterize a ring, the recognition of Lf.α by successive iterations.
doi: 10.1006/jmsc.1993.1015pmid: N/A
Greater silver smelt (Argentina silus) is an abundant benthopelagic species at depths greater that 200 m in the north-eastern North Sea and the Skagerrak. Mainly in April–May, adults concentrate to spawn in the deepest parts. Juveniles inhabit shallower marginal areas year-round. Some 30–35 age-groups occur and spawning concentrations are dominated by 20-year-old and older individuals. Age at first spawning, as determined from otoliths, is 4–9 years, usually 6 years. Females grow larger than males, asymptotic lengths being 42.6 and 40.3 cm, respectively.
Fryer, R. J.; Nicholson, M. D.
doi: 10.1006/jmsc.1993.1017pmid: N/A
Summarizing the temporal variation in an annual series of contaminant levels, for example as a downward or upward trend, is often complicated by the presence of an additional component of random year-to-year variation. This paper describes appropriate statistical tests for detecting two types of systematic variation, a linear trend and an incident in the presence of random between-year variation. Further, the effectiveness of a monitoring programme to detect these changes is quantified in terms of its statistical power. The results are used to assess the power of the International Council for the Exploration of the Sea's Co-operative Monitoring Programme of contaminant levels in fish muscle.
doi: 10.1006/jmsc.1993.1018pmid: N/A
The status of all known stocks of baleen whales that were severely depleted (to an estimated less than 10% of their original abundance) is reviewed. Of 44 such stocks, 18 are classified as not feasible to monitor. Of the remaining 26 stocks, 12 have been or are being monitored, and significant rates of increase have been demonstrated for 10 of them. At least 10 of the 16 monitorable stocks, for which a significant rate of increase has not been demonstrated, are believed to be increasing. The reasons for a lack of monitoring in most of these 16 stocks do not seem to be related to population size, and may reflect practical difficulties in obtaining representatives samples due to temporal or spatial segregation of the animals in relation to the study areas. In total, at least 77% of monitorable stocks are either believed or demonstrated to be increasing.Rates of increase measured in one bowhead, four right, one gray, one blue, and three humpback whale stocks range from 0.031 to 0.144. Estimates of stock depletion associated with these increase rates are only available for six stocks; the more depleted stocks show higher rates of increase than the less depleted. Under the “strong convexity” hypothesis for the relationship between sustainable yield rate and population size, the maximum sustainable yield rate (MSYR) is more than half the per capita increase rate at very low population sizes; this translates into MSYR values of 0.026 to 0.049 (mean 0.039) in seven of these severely depleted stocks. There remaining two stocks are believed to be at or slightly above 50% of their initial size, and thus in the vicinity of the maximum sustainable yield level for baleen whales as conventionally adopted by the International Whaling Commission: their increase rates (adjusted for hunting mortality) are 0.034 and 0.045. These data tend to support MSYR values (for these stocks) at the higher end of the range 0.01–0.04 (of total population size) as sometimes used in baleen whale assessments.
Brodziak, J. K. T.; Rosenberg, A. A.
doi: 10.1006/jmsc.1993.1019pmid: N/A
We develop an extension of the Leslie-DeLury model for estimating the abundance and exploitation rate of an annual squid stock described in Rosenberg et al. (1990 Fisheries Research, 8: 335–350). The extended model utilized research vessel survey or fishery catch per effort data to estimate fishing mortality as well as in-season fluctuations in the available stock size due to migration. Model parameters are estimated using multiple regression. The model is applied to the inshore Loligo pealei fishery off Cape Cod in the north-west Atlantic for the years 1983–1990. The results show the high degree of variability in the availability of squid to this fishery between years. There are marked differences in the fishing mortality and migration patterns in these 8 years which emphasizes the need for this type of modelling approach.
doi: 10.1006/jmsc.1993.1020pmid: N/A
During the 1980s the presence of a salinity minimum was noted in the water column of the Faroe-Shetland Channel at temperatures close to 0°C between Arctic intermediate (AI) water, formed along the Faroe-Iceland Ridge, and the Norwegian Sea deep (NSD) water occupying the lowest levels of the channel. The minimum was most marked in the north-east entrance to the channel, indicating a Norwegian Sea origin. Analysis of the long series of Nolsø-Flugga hydrographic sections shows a relatively large salinity variability which coincides with the depth of this minimum, suggesting that this water could have been present but undetected over many years. Similar water has been recently noted throughout the southern Norwegian Sea by Blindheim (1990 Deep-Sea Research, 37, 1474–1489), although his name for it, Arctic intermediate water, conflicts with the established use of this term for the warmer AI water found at lesser depths between northern Iceland and the Faroe-Shetland Channel. Hence the term Norwegian Sea intermediate (NSI) water is used here. Although modern CTD profiles greatly aid identification of this water, careful scrutiny of past water bottle sampling reveals evidence of its presence over the past three decades, the period for which high-quality salinity determinations have been available, showing that its presence is not solely a phenomenon of the 1980s. An additional indicator appears to be a dissolved silica minimum, which suggests a relatively recent connection with the upper waters. The minimum salinity of water between 0 and 1°C was therefore compared with the mean salinity of the upper north Atlantic (NA) water of the Faroe–Shetland Channel. A peak correlation at the 0.1 probability level was found to occur with a time-lag of 7 years between the salinity fluctuations of NA water passing into the Norwegian Sea and those appearing in the NSI water in the depths of the same channel.
Dunn, J.; Mitchell, R. B.; Urquhart, G. G.; Ritchie, B. J.
doi: 10.1006/jmsc.1993.1021pmid: N/A
The LOCHNESS (large opening closing high speed net and environmental sampling system) for quantitative sampling of micronekton is described. It consists of five nets, each 14 m long by 2.3 m square mouth opened, contained within a steel framework. The sampler is towed at a speed of 1.5-2.5 ms−1 and the nets are opened sequentially by remote control using a through-water acoustic command and telemetry system. Mounted on the sampler are sensors to indicate depth, water flow, battery voltage, and net status. This information is transmitted to the surface via the acoustic link and displayed in real-time on a visual display unit. The sampler also carries a separate, independent system to log depth, conductivity, temperature, and other parameters as required, which can be down-loaded to a computer on recovery. The sampler can thus collect biological samples and environmental data simultaneously. The engineering design, construction, and early trials are described and some initial results are presented showing both its mechanical and catching performance.
doi: 10.1006/jmsc.1993.1022pmid: N/A
The thickness of adult herring (Clupea harengus) otoliths was reduced chemically through selective decalcification in contrast to physical grinding. Microstructure around the nucleus was clearly visible under a light microscope after the preparation. The process of this technique included three steps: first, the two ridges along either side of the sulcus were roughly ground; second, the otolith exterior side was brought down to the midplane by decalcification; finally, the decalcification of the exterior side was prevented while the interior side was further decalcified to produce a thin section, which was then mounted on a glass slide and studied.
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