NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHOR(S) Northwest Atlantic Fisheries Organization (original) (raw)
ICES Journal of Marine Science, 2014
Since the late 1980s, a deep-sea fishery for Greenland halibut (Reinhardtius hippoglossoides) has been developing gradually in West Greenland. Deep-sea fish species are generally long-lived and characterized by late age of maturity, low fecundity, and slow growth, features that probably cause low resilience following overexploitation. In order to evaluate whether populations of nine potential bycatch species are negatively affected by the commercial fishery for Greenland halibut, scientific data from bottom-trawl surveys conducted in the same area and period as the commercial fishery were analysed. During the period 1988 -2011, population abundance and size composition changed as catch and effort in the Greenland halibut fishery increased. Two species showed a significant decrease in abundance, and four populations showed a significant reduction in mean weight of individuals (p , 0.05). Correlation analyses show that most of the observed trends in abundance are probably not related to increasing fishing effort for Greenland halibut. The analysis did, however, show that most of the observed decreases in mean weight were significantly correlated with fishing effort during the 24-year period.
Journal of Northwest Atlantic Fishery Science, 2003
The variation in population structure of Greenland halibut (Reinhardtius hippoglossoides Walbaum) in the Northeast Arctic is analysed using data from three different surveys using trawl, longline and gillnet gears, in the slope area of the western Barents Sea in the period 19922000. The design of the longline and gillnet survey had limitations in that they were set to simulate the commercial fisheries, but the vessels were forced to cover the most important part of the slope area. Greenland halibut was the dominant species both in numbers and weight and was usually caught in the range of 515 years old, but the catch was dominated by ages 612. The data showed differences in sex composition and age composition both by area and by depth, and catches from trawl showed the most evident pattern. Greenland halibut caught by gillnet were larger and older than fish caught by trawl and the results from longline were in between. In most of the age groups, males were significantly smaller than females and this general trend was shown for all gears. Males also dominated the younger age groups in all gears and Greenland halibut older than 10 years were virtually all females. Fish from the earliest cohorts in the years investigated were generally smaller than fish from later ones.
Fisheries Research, 2001
A joint Norwegian±Greenland longline survey was conducted at East Greenland in August 1997, using different hook and bait types. Most Norwegian longliners use hooks of the type EZ 12/0. This hook was compared to three versions of a new circle 14/0 hook. A total catch of 2899 Greenland halibut from 45,760 hooks baited with squid were used in the hook selectivity analyses. In average, CPUE was 281 kg/1000 hook for the EZ hook. CPUE for the circular hook was 36% higher making an overall signi®cant difference in CPUE between the EZ hook and the circle hooks. On 6630 hooks squid and grenadier were used alternately. The CPUE of Greenland halibut was 25% higher for grenadier bait. The grenadier bait resulted in a reduction in bycatch compared to the squid bait (1.1 and 20.7% by numbers, respectively). Catches by EZ 12/0 hook and one of the circle 14/0 hooks were compared in order to examine size selectivity. Using the SELECT approach, expected proportions were ®tted to the observed proportions for ®ve different models of selectivity. All models resulted in almost identical ®ts. The absence of non-selective data requires the choice of selectivity curve to be based on knowledge about the capture process. Since the selectivity curves cannot be determined unambiguously in this study, none of the estimated curves are preferred for the other.
22 s., 1997
Based on annua! bottom trawl surveys in the Barents Sea and Svalbard area in 1983-96, the paper describes variations in abundance and composition of 1-group Greenland halibut (Reinhardtius hippoglossoides). The main pattem of variation in abundance durit:tg the period .. was a change from high abundance in the first half of the period, to low abundance in the • • second half. The reduced abundance was associated with a northerly shift in distribution. In • the låSt two years the recruits reappeared, first in northem parts of the study area and then also . further south.. These periodical changes in juvenile distribution, may be driven .by physical • • oceanog~aphical processes. Distribution changes probably extend beyond the area covered by the annua! surveys. Thus, the results add uncertainty to the resent assessi.nen~~ of the state of the stock: •
Bulletin of Marine Science, 2016
In data-limited fisheries, making informed management decisions based on scientific advice is challenging. Here, we evaluate a multi-indicator adaptive management framework (AMF) that allows dynamic responses to changing environmental, socioeconomic, and fishing conditions. Using stakeholder-defined goals as a foundation for specifying performance metrics, we employ management strategy evaluation (MSE) to explore the performance of the AMF relative to prescriptive alternatives that are sometimes used in data-limited situations. We conduct simulations involving the two most economicallyimportant fisheries in Belize, spiny lobster, Panulirus argus (Latreille, 1804), and queen conch, Strombus gigas (Linnaeus, 1758). Spiny lobster fishery simulations demonstrate that when relatively stable catches have historically persisted, an AMF can help to ensure that stable catches continue to persist into the foreseeable future when faced with factors such as increased entry to the fishery or environmentallyinduced recruitment fluctuations. The queen conch fishery simulations demonstrate that optimizing economic performance is complicated without stock status indicators and depends greatly upon the current, yet typically unknown, state of the resource. Since our indicator-based approach could not provide direct information about resource status in relation to management reference points such as maximum sustainable yield, economic objectives could not be achieved. Nevertheless, implementing the AMF served as a beneficial control against stock collapse and could function well as an interim fishery policy during which sufficient fishery data could be collected to inform population modeling and quantitative stock assessment.
2007 Report of the ICES Study Group on Fisheries-Induced Adaptive Change (SGFIAC)
2007
There is a growing body of scientific evidence indicating that fisheries can cause evolutionary responses over time periods as short as 1020 years, in particular in traits such as the onset of maturation. As these changes will most likely result in a reduction of the productivity of a fish stock, management objectives and (precautionary) reference points for sustainable exploitation need to be re-defined, and new objectives and reference points for managing fisheries-induced evolution need to be developed. Current knowledge allows for two generalisations. First, reducing harvest rates will almost always slow the rate and extent of fisheries-induced evolution in most life-history traits. Second, raising a stock's minimum size limit for exploitation well above the size range over which maturation occurs will slow down the rate of evolution in its maturation schedule. To go beyond these generic insights, "Evolutionary Impact Assessments" (EvoIAs) are proposed to quantify ...
A simple implementation of the statistical modelling framework Gadget for cod in Icelandic waters
African Journal of Marine Science, 2007
Gadget is a statistical modelling framework that can be used to assess individual fish stocks and to create multispecies, multi-fleet and multi-area models. The development of a complex multispecies model requires understanding of the modelled single-species population. Simple single-species models are essential in evaluating whether more complex models increase understanding of the population dynamics. In this paper, Gadget is illustrated using a single-species case study with two stock components on one area. Features of the implementation include immature and mature stock components, a maturation process, along with commercial and survey fleets. Parameter estimation is done using maximum likelihood based on a variety of datasets. The estimated parameters relate to growth, maturation, fleet selection, recruitment and the initial population. The data types used to calculate the likelihood include survey indices and biological samples from the commercial catch and surveys. Optimisation of the model, along with a protocol to estimate appropriate weighting of the likelihood components, is described. A range of different assumptions and estimation methods are evaluated including the effect of reduced data availability on parameter estimation by excluding age data from the likelihood components.
Evaluating the estimation of fishery management reference points in a variable environment
Fisheries Research, 2009
There is strong evidence that low frequency between-year environmental variability, in addition to fishing, is able to affect fish population abundance via recruitment. However, scientific advice regarding catch limits is often based on control rules that depend on the estimation of biomass reference points which typically do not explicitly consider the effects of trends over time in reference points caused by environmental variability. Harvest rates based on commonly used biological reference points such as the level of unfished spawning biomass (B 0 ), the current size of the stock in relation to B 0 , and B MSY that are sustainable under current environmental conditions may be unsustainable under different environmental conditions. Although several methods exist for estimating biomass reference points, it is unclear which of these are most robust to the effects of long term, low frequency environmental variability. Therefore, simulation is used to evaluate alternative estimators, which differ in terms of how the stock-recruitment relationship is modeled, and whether explicit estimators or proxies are used for B 0 , the steepness of the stock-recruitment relationship , and current spawning biomass relative to B 0 . The simulations consider three life histories: a long-lived unproductive rockfish, a moderately long-lived and productive flatfish, and a moderately long-lived and productive hake with highly variable recruitment. Results indicate that in the presence of low frequency autocorrelated forcing of recruitment, biomass reference points should be based on average recruitment and/or dynamic B 0 if catch and survey data are available for at least one full period of the environmental variable. In contrast, previous analysis suggests that in the absence of autocorrelated environmental forcing of recruitment, and if the available catch and survey data do not span at least, in this case, 50 years which is one full period of the environmental variable, biomass reference points should be based on the fit of the stock-recruitment relationship. Life history affects the estimability of biomass reference points, which are more difficult to estimate for species with more rapid dynamics such as hake. The method used to calculate the reference points given the results of a stock assessment has a larger effect on estimability than the configuration of the stock assessment method, for the three stock assessment model configurations investigated in this study.