ICES Journal of Marine Science: Journal du Conseil

Chereskin, T. K.; Tarling, G. A.

doi: 10.1093/icesjms/fsm138pmid: N/A

Chereskin, T. K., and Tarling, G. A. 2007. Interannual to diurnal variability in the near-surface scattering layer in Drake Passage. – ICES Journal of Marine Science, 64: 1617–1626.Backscattering strength was estimated from 127 shipboard surveys with an acoustic Doppler current profiler (ADCP) made during Drake Passage transits from 1999 to 2004. The backscattering strength is used to determine the characteristics of the near-surface scattering layer, which south of the Southern Antarctic Circumpolar Current Front (SACCF) is dominated by Antarctic krill (Euphausia superba). Diel vertical migration in the upper 150 m was the dominant variability observed in any single transect. When averaged over depth, there was a well-defined annual cycle in backscattering strength, with a factor of four increase from a late-winter minimum to a spring-summer maximum over a period of four months, followed by a more gentle decline during late summer and autumn. In addition, there were significant differences in scattering strength north and south of the Polar Front (PF) on both seasonal and interannnual time-scales. The average summer maximum to the north of the PF was more than twice the maximum to the south, but the winter minima were about the same. On interannual time-scales, scattering strength south of the PF displayed a negative linear trend primarily attributable to a fourfold decrease in backscattering strength south of the SACCF. No significant long-term trend in the scattering strength north of the SACCF was observed.

Petihakis, G.; Smith, C. J.; Triantafyllou, G.; Sourlantzis, G.; Papadopoulou, K-N.; Pollani, A.; Korres, G.

doi: 10.1093/icesjms/fsm161pmid: N/A

Petihakis, G., Smith, C. J., Triantafyllou, G., Sourlantzis, G., Papadopoulou, K-N., Pollani, A., and Korres, G. 2007. Scenario testing of fisheries management strategies using a high resolution ERSEMPOM ecosystem model. ICES Journal of Marine Science, 64: 16271640.Ecosystem models are just beginning to be considered as management tools. In terms of fishery impacts, dynamic ecosystem models provide an opportunity to make advances because they can both evaluate the state of the system and make predictions about the ecosystem under various fishing scenarios. In the framework of the Cost Impact project, a complex high-resolution (500 500 m grid) ecosystem model was implemented in Iraklion Bay, Crete. Several management scenarios were simulated to investigate the impacts of trawling on this particular ecosystem (reductions in fishing area, effort, and mortality). Introducing trawling impacts into the model led to increases in pelagic production. All scenarios also resulted in net increases in pelagic production, the level of which, and the degree of spatial variability, was dependent on the particular scenario. Changes in pelagic variables were often noted in areas well away from trawled areas. It was also clear that for pelagic variables and processes, depth of trawling is more important than reduction in trawling area, i.e. a scenario banning fishing in waters shallower than 100 m seems to lead to less change in the pelagic system than a scenario that reduces direct mortality to the benthos.

Dickey-Collas, Mark; Pastoors, Martin A.; van Keeken, Olvin A.

doi: 10.1093/icesjms/fsm155pmid: N/A

Dickey-Collas, M., Pastoors, M. A, and van Keeken, O. A. 2007. Precisely wrong or vaguely right: simulations of noisy discard data and trends in fishing effort being included in the stock assessment of North Sea plaice. ICES Journal of Marine Science, 64: 000000.ICES stock assessments of North Sea plaice are routinely carried out with eXtended Survivors Analysis (XSA), based on landings and survey data. Recently, the assessments included data on discarded young fish, sampled with high variance. Fishing effort has been declining since the mid-1990s, so conditioning the estimated fishing mortality (F) on the recent past could introduce bias into the perceived stock size. Simulated populations with North Sea plaice-like characteristics are used to explore the dependence of the perceived stock dynamics on the inclusion of discards data at different sampling noise, using the same methods and XSA settings as ICES. The sensitivities of the results were tested against different trends in fishing effort and recruitment, and different scenarios for shrinkage (i.e. the way in which the past is used to estimate the most recent fishing mortality). Within the bounds of the simulation assumptions, the perception of population trends from an XSA stock assessment can be biased when there are trends in fishing effort: decreasing effort leads to underestimating SSB and overestimating F. When discards are not included, bias in SSB is greatest when effort decreases, and bias in F is greatest when effort increases. Bias in SSB and F were removed by including discard data, but at substantial loss of precision. If effort shows a clear trend and discards are substantial and estimated noisily, the recent trend in the target population may be hard to track with an XSA-type assessment methodology.

Heath, Michael R.

doi: 10.1093/icesjms/fsm156pmid: N/A

Heath, M. R. 2007. The consumption of zooplankton by early life stages of fish in the North Sea. ICES Journal of Marine Science, 64: 16501663.Previous work has shown that during the 1970s, fish and carnivorous macrozooplankton together consumed 22 gC m2 year1 of mesozooplankton, principally copepods. Consumption declined to 17 gC m2 year1 during the 1990s, mainly because of a reduction in fish production. The zooplankton production required to meet this demand seems to be approximately accounted for by estimates of new primary production, but there are additional sinks for zooplankton production attributable to predation by, for example, gelatinous species. Additionally, the consumption of zooplankton by early life stages of fish is difficult to assess and could be larger than implied by the earlier analysis. Here, the role of fish early life stages in zooplankton consumption is re-assessed, and found to be approximately double that previously estimated. Some 28% of the zooplankton consumption by fish is now estimated to be attributable to early life stages, resulting in an estimate of zooplankton consumption by the fish community as a whole 14% higher. Taken overall, the consumption of zooplankton production by fish and other planktivorous predators is now estimated to be 1925 gC m2 year1.

Cutter, George R.; Demer, David A.

doi: 10.1093/icesjms/fsm151pmid: N/A

Cutter, G. R. Jr and Demer, D. A. 2007. Accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys. ICES Journal of Marine Science, 64.Multibeam echosounders can improve the efficiency and the precision of acoustic-survey estimates by providing greater sampling volumes than single-beam echosounders. For a multibeam echosounder, the target strength of fish can vary with its pitch, roll, and yaw. Here, normalized, acoustic backscatter patterns from fish schools are modelled by beam-incidence angle,0180, considering the scattering-directivity patterns of each fish. Variation of pitch angle causes a decrease in the mean and an increase in the variance of the backscatter in the vertical beam, but has no effect on the backscatter in the outer beams. Conversely, variation of the yaw causes a decrease in the mean and an increase in the variance of the backscatter in only the outer beams. Because the fish-scattering model predicts different backscatter at dorsal- vs. lateral-incidence angles, backscatter did vary with roll angle. In the hypothetical case of fish avoiding a vessel, the backscatter decreases strongly the outer beams. The results of the model were compared with multibeam measurements of fish schools. In general, the measured mean backscatter vs. beam-incidence angle was nearly uniform. The methods described here provide an approach to accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys.

Barrett, Robert T.; Camphuysen, Kees (C. J.); Anker-Nilssen, Tycho; Chardine, John W.; Furness, Robert W.; Garthe, Stefan; Hppop, Ommo; Leopold, Mardik F.; Montevecchi, William A.; Veit, Richard R.

doi: 10.1093/icesjms/fsm152pmid:

Smith, C. J.; Banks, A. C.; Papadopoulou, K-N.

doi: 10.1093/icesjms/fsm165pmid: N/A

Smith, C. J., Banks, A. C., and Papadopoulou, K.-N. 2007. Improving the quantitative estimation of trawling impacts from sidescan-sonar and underwater-video imagery. ICES Journal of Marine Science, 64: 16921701.The techniques of sidescan sonar and towed, underwater-video sled were assessed as rapid-assessment methodologies for investigating trawl impacts on the substratum. Sidescan sonar is able to image a swathe of 200 m with a resolution of 20 cm at a speed of 23 knots, and marks of trawl doors could be observed. The towed video system imaged a swathe of 12 m with a resolution of 12 cm at a speed of 1 knot, and trawl-door marks, scrape marks, local bioturbation features, and fauna could be observed. Multiple tows using both methodologies were carried out in two areas in Heraklion Bay, Crete. One area, experimentally trawled, was 8090 m deep and characterized by mixed, maerly sediments; the other was a commercial trawl lane 200 m deep characterized by silty-clay sediment. Descriptions of the types of trawling feature and impacts caused by trawling were made for both areas. Images were analysed from the commercial deeper trawling ground for area assessment. For sidescan-sonar records, direction of trawling and trawl-mark density by category were estimated at periodic intervals along the track. For video, categories for trawl-mark density and level of bioturbation were estimated, along with the density of the crinoid Leptometra phalangium. Using geo-referenced positioning for each data point, area maps were constructed for each of the parameters, and correlations were tested between the different datasets. The use of the assessment techniques (characteristics, data usage, mapping, complementarity) in relation to trawling-impact studies is discussed, as well as possibilities for the use of the resulting data for management.

McAuley, R. B.; Simpfendorfer, C. A.; Wright, I. W.

doi: 10.1093/icesjms/fsm136pmid: N/A

McAuley, R. B., Simpfendorfer, C. A., and Wright, I. W. 2007. Gillnet mesh selectivity of the sandbar shark (Carcharhinus plumbeus): implications for fisheries management. ICES Journal of Marine Science, 64.Gillnet mesh selectivity parameters for the sandbar shark (Carcharhinus plumbeus) were estimated from catches taken by an experimental net of six panels of mesh, varying in size from 10.2 to 25.4 cm. The length selectivity of each mesh size was described by five different models. According to model deviance values, the four models based on the SELECT method of estimation provided better fits to the data than the gamma model previously applied to sharks. Lengths at maximum selectivity were estimated to be between 5.3 and 7.0stretched mesh size. The breadth of the selectivity curves was greater than have been reported for most species of shark. Lognormal and normal curve forms yielded the lowest model deviance and were judged to provide the best fits to the data. Peak selectivity of the commercially utilized mesh sizes was generally estimated to be greater than the observed modal length class of the commercial C. plumbeus catch. This suggests that a relatively high abundance of smaller sharks in the study area offsets gear selectivity effects in determining the size composition of commercial catches. These results have important implications for the recovery of this overexploited stock and also for managing international gillnet fisheries for the species.

McAuley, Rory B.; Simpfendorfer, Colin A.; Hall, Norm G.

doi: 10.1093/icesjms/fsm146pmid: N/A

McAuley, R. B., Simpfendorfer, C. A., and Hall, N. G. 2007. A method for evaluating the impacts of fishing mortality and stochastic influences on the demography of two long-lived shark stocks. ICES Journal of Marine Science, 64.Stochastic demographic models were developed for Carcharhinus obscurus and C. plumbeus populations off the west coast of Australia by resampling the input parameters for life tables from empirical biological data collected from commercial target fisheries and fishery-independent surveys. The models were used to examine the effects of multiple scenarios of age-specific survival, derived from the fishing mortality rates estimated from a tagging study on sharks and indirect estimates of natural mortality. In the absence of fishing, median estimates of the rates of intrinsic population increase (r) were 0.025 for both species. Inclusion of the age-specific fishing mortality rates estimated for C. obscurus recruits born in 1994 and 1995 resulted in the median estimates of r declining to 0.007 and 0.012, respectively, suggesting that recent harvest levels of mainly neonates by the target fishery were probably sustainable. However, the model also suggested that the population was more susceptible to exploitation of older sharks than was previously believed. The C. plumbeus model indicated that fishing mortality between 2001 and 2004 was probably unsustainable. The increasingly negative trend in median r estimates (from 0.032 to 0.049), and the populations apparently limited capacity for density-dependent compensation through changes in fecundity, somatic growth and longevity, suggests that management intervention is necessary to prevent continued stock depletion.

Roa-Ureta, Rubn; Niklitschek, Edwin

doi: 10.1093/icesjms/fsm149pmid: N/A

Roa-Ureta, R., and Niklitschek, E. 2007. Biomass estimation from surveys with likelihood-based geostatistics. ICES Journal of Marine Science, 64.A likelihood-based geostatistical method for estimating fish biomass from survey data is presented. Biomass estimates from analysis of a positive random variable with an additional discrete probability mass at zero means that the method accommodates null observations and positive fish density. The positive fish density data were used to estimate mean fish density in the subareas where the stock was present. A presence/absence representation of the data in the survey area was modelled with a generalized linear spatial model of the binomial family, leading to an estimate of the area effectively occupied by the stock. As an extension, a procedure is proposed to accommodate extra sources of correlation, such as multiple surveys or multiple vessels. The new methodology was applied to three cases. The simplest case is a scallop trawl survey for which only the positive density data need to be analysed. The intermediate case is a trawl survey of highly mobile squid where the stock area and the mean density inside the stock area are analysed. The most complex case is in estimating the biomass of very localized orange roughy, for which repeat surveys create dependence in the data in addition to spatial correlation.