ICES Journal of Marine Science: Journal du Conseil

Greenstreet, Simon P.R.; Rogers, Stuart I.

doi: 10.1016/j.icesjms.2005.12.009pmid: N/A

The shift in emphasis away from the single-species focus of traditional fisheries management towards an ecosystem approach to management requires application of indicators of ecosystem state. Further, an ecosystem approach to management requires the identification of ecological reference points against which management objectives might be set. In applying indicators, identifying reference points, and setting objectives, an obvious requirement is that the indicators respond primarily to the anthropogenic activity being managed and are sufficiently sensitive that impacts of the activity and the responses to management action are clearly demonstrable. Here we apply a suite of 12 indicators to Scottish August groundfish survey data collected in the northern North Sea over the period 1925–1997. These include indicators of size structure, life-history character composition, species diversity, and trophic structure within the community. Our choice of analytical design has two purposes; first to show that fishing has unequivocally affected these various aspects of the structure of the groundfish community, and second to illustrate an approach by which long time-series data sets might be used to identify possible management reference points. The results are discussed in the context of selecting ecological indicators in support of an ecosystem approach to management and determining appropriate reference points for objective-setting.

Francis, R.I.C. Chris

doi: 10.1016/j.icesjms.2006.01.001pmid: N/A

There has recently been considerable interest in establishing relationships between environmental variables and annual recruitment to fish stocks. Such relationships have the potential to reduce the uncertainty in the assessment of the stocks. When many environmental variables are considered, it is easy to draw conclusions that exaggerate the ability to predict recruitment. One technique to protect against this is cross-validation. This technique has usually been incorrectly applied, in that it has not included predictor screening (the selection from a large set of potential predictors of a smaller set to use in prediction). A simulation experiment is used to show that this omission can cause chance correlations to be wrongly identified as useful, and the reliability of useful predictors to be overestimated. It also shows that the mistaken use of chance correlations to predict recruitment can be worse than the use of the default predictor (the mean of previous recruitments), and that our ability to measure the reliability of recruitment predictors is typically poor.

Kelly, Ciaran J.; Codling, Edward A.; Rogan, Emer

doi: 10.1016/j.icesjms.2005.12.001pmid: N/A

Historically, cod has been one of the most important fish stocks in the North Atlantic. Recent stock collapses have been attributed to overfishing, and in February 2000 the European Commission established a closed area in the Irish Sea as part of a general recovery plan. The recovery plan was further revised and implemented between 2001 and 2005. However, the recovery plan has not provided the expected benefit, and the stock is still thought to be below the safe limit of Blim = 6000 t. We use stochastic simulations to investigate possible stock trajectories over a seven-year period from 1999 to 2005 under various scenarios of fishing mortality. Comparing the results of simulations with reality, it is clear that more drastic action is required if the stock is to recover in the medium term. The recovery plan was not explicitly designed to deal with uncertainty in the system, and this, we believe, resulted in the failure to meet the recovery plan objectives.

Link, P. Michael; Tol, Richard S.J.

doi: 10.1016/j.icesjms.2006.01.003pmid: N/A

A bioeconomic simulation model of the two interacting fish species cod (Gadus morhua) and capelin (Mallotus villosus) and their fisheries is presented and applied to assess the consequences of changes in the population dynamics of these important fish stocks in the Barents Sea. In each scenario, the population dynamics of the fish species are changed by reducing the reproduction-induced productivities and/or the carrying capacities. Stock sizes and landings of fish are calculated for each fishing period, and the net present values of profits from fishing are determined for time periods prior to and after the change in population dynamics. Results show that reduced growth rates or carrying capacities lead to lower stock levels and consequently to smaller catches. There is only a small short-term economic impact on the fisheries, but the long-term consequences are pronounced. In some cases, greater fishing activity in the first few years after the change in population dynamics causes harvests to remain stable despite diminishing stock sizes. This stabilizes the returns from fishing in the short term, but veils the apparent negative long-term impact on the fisheries resulting from adversely affected stock dynamics.

Borja, Ángel; Liria, Pedro; Muxika, Iñigo; Bald, Juan

doi: 10.1016/j.icesjms.2005.12.008pmid: N/A

Marine protected areas play an important role in the conservation of marine resources and fisheries management. In the Basque Country (northern Spain), the small (158 ha) Gaztelugatxe Marine Reserve was established in 1998; after 5 years without exploitation, it is considered likely that the goose barnacle Pollicipes pollicipes populations have recovered. This contribution provides information on the standing stock, and of the relationships between biomass, coverage, and density, and environmental factors such as wave height and energy derived from waves received at the coast. Increasing energy levels produce enhanced coverage and abundance, providing a bigger standing stock in the reserve. Numerical models to simulate the energy produced by waves can be used as a tool to predict the potential biomass of the goose barnacle along the coast. Such an approach allows comparison of observed and predicted biomasses, and possibly also determination of the factors involved in the observed differences, e.g. overexploitation and pollution; this will assist in assessing goose barnacle resources along the coast.

Cook, E.J.; Black, K.D.; Sayer, M.D.J.; Cromey, C.J.; Angel, D.L.; Spanier, E.; Tsemel, A.; Katz, T.; Eden, N.; Karakassis, I.; Tsapakis, M.; Apostolaki, E.T.; Malej, A.

Giannoulaki, Marianna; Machias, Athanassios; Koutsikopoulos, Constantin; Somarakis, Stylianos

doi: 10.1016/j.icesjms.2005.10.017pmid: N/A

Acoustic-survey data from 1995 to 2004 (six acoustic surveys in summer and two in winter) in the Aegean and Ionian Seas (eastern Mediterranean Sea) were analysed to investigate the spatial organization of European anchovy and European sardine populations. The potential effect of certain topographic characteristics (e.g. area, bottom depth, and the degree of land enclosure) on the spatial structure of the fish was studied in different geographic subareas (i.e. how topography affects the organization of fish into clusters of schools). Parameters calculated by geostatistical techniques were used as descriptors of the spatial organization. The results indicate the significant effect of area and land enclosure on the spatial structures of both species, suggesting that environmental spatial heterogeneity attributable to coastal topography affected the way fish schools were organized into aggregations. In summer, the spatial structure of sardine was more heterogeneous in subareas with increased land enclosure, whereas the spatial structure of anchovy was not significantly related to any of the area characteristics examined. In winter, the spatial structure of both species was more heterogeneous in subareas with increased enclosure and in small rather than larger subareas. The findings are discussed in terms of the species' response to their environment.

Silva, A.; Santos, M.B.; Caneco, B.; Pestana, G.; Porteiro, C.; Carrera, P.; Stratoudakis, Y.

doi: 10.1016/j.icesjms.2006.01.005pmid: N/A

We investigate spatial and temporal variations in sardine maturation patterns and discuss the implications for stock structure, using recent samples collected opportunistically across the species range and data collected regularly for the assessment of the Atlanto-Iberian stock. Maturity ogives were fitted to maturity-at-length data using Generalized Linear Models, and maturation patterns are summarized by length at first maturity (L50). Sardine attain sexual maturity at a total length of 10.9–16.8 cm. The percentage of mature small sardine increases from north to south across the Northeast Atlantic. L50 is low in the Mediterranean and Moroccan waters, but maturation is delayed towards the limits of the species' range. Sardine mature smaller in the Gulf of Cadiz than in other areas of the Atlanto-Iberian stock, although there is no indication of a different maturation pattern north of the stock boundary. These results are not sufficient to review the stock structure of sardine, but question the hypothesis of homogeneous biological properties of sardine populations within the stock area. Sardine maturation length reduced and the spawning period extended in western Iberia during the past two decades. Associated increases in fish condition and fat reserves during the summer feeding season suggest that variations in reproductive traits may have been caused by environmentally driven changes in food availability.

Tenningen, Eirik; Churnside, James H.; Slotte, Aril; Wilson, James J.

doi: 10.1016/j.icesjms.2005.11.018pmid: N/A

A linearly polarized green (532 nm) laser and a digital video camera were used to determine the reflectivity (R) and lidar (LIght Detection And Ranging) target strength (TS) of live mackerel by comparison with a standard calibration target. The measured reflectivity was 0.0141 ± 0.0005 when the receiver was copolarized with the laser and 0.0092 ± 0.0004 when the receiver was cross-polarized. The corresponding TS values were −42.66 ± 0.24 dB for the copolarized channel and −44.86 ± 0.23 dB for the cross-polarized channel. The depolarization ratio (depolarized return over total return) of 0.396 is very different from earlier measurements of sardine, suggesting that depolarization might be useful for species identification.

Yasuma, Hiroki; Takao, Yoshimi; Sawada, Kouichi; Miyashita, Kazushi; Aoki, Ichiro

doi: 10.1016/j.icesjms.2005.02.016pmid: N/A

This paper reports theoretical values of target strength (TS) for the lanternfish Stenobrachius leucopsarus, a fish without an airbladder, which dominates the Subarctic marine mesopelagic fish community. Two models for liquid-like slender bodies, the general prolate-spheroid model (PSM) and the deformed-cylinder model (DCM), were used to compute the TS of the fish relative to its orientation. The relative mass density g and the sound speed h in seawater were measured and used in both models. To confirm the appropriateness of the models, tethered experimental measurements were carried out at 38 kHz for five specimens. The value of g measured by the density-bottle method was very low (1.002–1.009) compared with that of marine fish in general. The value of h measured by the time-average approach was 1.032–1.039 at the water temperature at which S. leucopsarus is found. TS-fluctuation patterns against fish orientation (the TS pattern) estimated from the DCM and PSM were in good agreement in the area of their main lobes. Both models reproduced the main lobes of the measured TS patterns in near-horizontal orientation (<±20°), and they were considered to be effective in measuring the TS of S. leucopsarus in a horizontal (swimming) position. After these comparative experiments, we computed the TS of 57 fish (27.8–106.9 mm) at 38, 70, 120, and 200 kHz, using the DCM. A plot of body length (in log scale) against TS showed a non-linear relationship at all frequencies. S. leucopsarus had a very low TS (<−85 dB, TScm), suggesting that acoustic assessment would be highly sensitive, especially when the proportion of small fish is high (e.g. L/λ < 2), and an appropriate frequency should be considered that takes into account both the length composition and the depth of occurrence.