CCAMLR

The depletion-corrected average catch (DCAC) formula is an extension of the potential-yield formula, and it provides useful estimates of sustainable yield for data-poor fisheries on long-lived species. Over an extended period (e.g. a decade or more), the catch is divided into a sustainable yield component and an unsustainable “windfall” component associated with a one-time reduction in stock biomass. The size of the windfall is expressed as being equivalent to a number of years of sustainable production, in the form of a “windfall ratio”. The DCAC is calculated as the sum of catches divided by the sum of the number of years in the catch series and this windfall ratio. Input information includes the sum of catches and associated number of years, the relative reduction in biomass during that period, the natural mortality rate (M, which should be ,0.2 year21), and the assumed ratio of FMSY to M. These input values are expected to be approximate, and based on the estimates of their imprecision, the uncertainty can be integrated by Monte Carlo exploration of DCAC values.

We describe a method for determining reasonable yield and management reference points for data-poor fisheries in cases where approximate catches are known from the beginning of exploitation. The method, called Depletion-Based Stock Reduction Analysis (DB-SRA), merges stochastic Stock-Reduction Analysis with Depletion-Corrected Average Catch. Data requirements include estimates of historical annual catches, approximate natural mortality rate and age at maturity. A production function is specified based on general fishery knowledge of the relative location of maximum productivity and the relationship of MSY fishing rate to the natural mortality rate. This leaves unfished biomass as the only unknown parameter, which can be estimated given a designated relative depletion level near the end of the time series. The method produces probability distributions of management reference points concerning yield and biomass. Uncertainties in natural mortality, stock dynamics, optimal harvest rates, and recent stock status are incorporated using Monte Carlo exploration. Comparison of model outputs to data-rich stock assessments suggests that the method is effective for estimating sustainable yields for data-poor stocks.
 

The Magnuson-Stevens Fishery Conservation and Management Reauthorization Act of 2006 requires Regional Fishery Management Councils to set annual catch limits for all stocks or stock complexes in Federal fishery management plans beginning in 2011. Most species listed in the Pacific Coast Groundfish Fishery Management Plan have not been assessed, in large part due to data limitations. Estimates of sustainable yield for many these species were previously based on undocumented, ad-hoc analyses. We present estimates of sustainable yield for 50 of these stocks using two recently developed models designed to inform management of data-poor stocks. These models rely on recently reconstructed time series of historical catch for west-coast groundfish species and species-specific information related to stock productivity. For this set of data-poor stocks, recent landings statistics reflect shifts in the relative importance of certain species to west-coast fisheries (e.g. increased catches of nearshore and slope rockfish species relative to shelf species), largely due to recent regulatory actions. We provide estimates of overfishing limits (OFLs) for each of the 50 stocks along with comparisons to recent catch levels. Our results suggest that status quo harvest levels range from light exploitation of some stocks to potential overfishing of others. This information could help inform decisions regarding prioritization of future stock assessments for unassessed species. OFLs are expressed as probability distributions, reflecting our uncertainty in model parameters. We select median values as point estimates of OFL, as this statistic is most consistent with National Standard 1 guidelines.
 

The Kerguelen Plateau (Statistical Divisions 58.51 and 58.5.2) supports the largest Patagonian toothfish (Dissostichus eleginoides) fishery in the Southern Ocean outside of the Atlantic sector. Analysis of genetic, demographic and tagging data indicates that toothfish form a metapopulation in this region. This paper describes the development of a CASAL model incorporating data from research, commercial and illegal fishing activities in the French and Australian EEZs. This represents a substantial step forward in understanding the dynamics and current status of toothfish across the Kerguelen Plateau. Preliminary results indicate that illegal fishing in the late 1990s and early 2000s lead to a rapid decline in stock status, however action to eliminate illegal has moderated this decline. Model fits to the substantial decline in catch rates seen in the French EEZ concurrent with illegal activities are poor, indicating that the SSB0 and stock status estimates still need to be interpreted with caution. Continuing pre-recruit surveys across the Australian and French EEZ, ageing samples from surveys and the commercial catch, and modelling tag and demographic data to better understand rates of movement and the distribution and abundance of age classes across the Plateau are suggested as part of a research program to addressing key uncertainties. Such work would enable the development of models that more fully capture the complex dynamics of toothfish throughout their life on the Kerguelen Plateau.
 

Exploratory research voyage of F/V 'TAMANGO' proceeded from March, 23 to May, 8, 2010. Three areas were explored during the voyage: Patagonian shelf (Statistic Subarea 41.3.1), South Orkney Islands shelf (CCAMLR Subarea 48.2) and the North Scotia Ridge. Exploratory pot-line survey revealed no commercial concentrations of crabs on the South Orkney Islands shelf. However, commercial concentrations of king crabs were encountered in the area adjacent to CCAMLR area, on the North Scotia Ridge, at depths ranging from 600 m to 1400 m. Turkayi king crab (Lithodes turkayi) and stone king crab (Neolithodes diomedeae) were the main commercial crab species on the North Scotia Ridge. During the voyage, three species of king crabs were found on the Patagonian shelf: Lithodes santolla, Paralomis spinosissima и Paralomis Formosa. One species (Paralomis formosa) was found in waters around the South Orkney Islands. Four species of king crabs were found on the North Scotia Ridge: Lithodes confundens, Lithodes turkayi, Neolithodes diomedeae и Paralomis formosa.
 

The rate at which tags are lost from tagged toothfish is an important parameter in modelling of the Antarctic toothfish (Dissostichus mawsoni) populations in Subarea 88.1 and 88.2. The toothfish stocks in these areas have been assessed using data from tag-release and recapture experiments within a CASAL integrated stock assessment model, using tag loss rates derived by Dunn et al. (2005). We update their estimates and calculate tag loss rates for both single and double tagged fish for use within the CASAL stock assessment models from the available data.
Revised estimates of the rate at which individual tags are lost from tagged toothfish in Subareas 88.1 and 88.2 from a sample of 969 double tagged and subsequently recaptured fish suggested that the loss rate was about 3.5% (95% C.I.s 0.020–0.054) of individual tags were lost almost immediately, and then there was an ongoing rate of about 0.039 (95% C.I.s 0.027–0.052) tags per year. For double tagged fish this corresponds to 99.5% of double tagged fish having at least one tag remaining after one year at liberty; 98.9% after two years at liberty; declining to 94.6% after six years; and to 88.4% after ten years.
Comparison of the different loss rate models suggested that there was evidence of immediate failure of tags (α = 0.035) and an ongoing constant rate of failure (λ = 0.039 y-¹), but no evidence of a change in the failure rate over time. It is plausible that there could be a catastrophic failure of tags or some other systematic change in the tag loss rate after some long period at liberty, but there was no evidence of such failure in these data for periods of up to six years at liberty.
The loss rate for double tags had been incorrectly derived and applied in the assessment models of Dunn & Hanchet (2009a, 2009b), with the loss rates slightly over-estimated for double tagged fish in the first four years and under-estimated after that. However, the tag loss rate and the double tag approximation rates calculated in this study suggest that the change in value of the tag loss rate parameter combined with the incorrect double tag model had very little impact on the assessment estimates of biomass in the assessment models.
The equivalent tag loss rate that can be used to provide a close approximation of the true tag loss rate in the Subarea 88.1 and 88.2 assessment models is either λ = 0.0071 y-¹ (where we exclude recapture events that occur after four years) or λ = 0.0084 y-¹ (where we exclude recapture events that occur after six years). Simulations showed that the impact on the assessment of ignoring tag recapture data after a six year period was to introduce negligible bias of less than 0.5% with less than 1% change in the overall estimated variance (mean squared error).  Similarly, a simulation experiment that excluded earlier years of release data from the assessment models  also suggested that the removal of early data had little impact on the assessment models. Estimates of bias were negligible (<0.5%) and the increased variability (as measured by mean squared error) was less than 3% even if all tag release and recapture data before 2005 were removed.
 

Antarctic and Patagonian toothfish stocks in the Antarctic have been assessed within CCAMLR areas using integrated statistical catch-at-age stock assessment models. These models were initially run annually, but have been run biennially since 2007. This document investigates the impact on model outputs of withholding from a stock assessment between one and eight years of age frequency and/or tag recapture data. Examining the impact of considering a subset of the available data simulates both the effect of not updating an assessment every year, and also the evolution of stock assessments with time as more years’ data becomes available.
The 2009 stock assessment of Antarctic toothfish in the Ross Sea region was used as the working model. The 2009 stock assessment of Antarctic toothfish in Subarea 88.2E was used as a sensitivity to examine the impact of withholding data from an assessment based on a smaller and shorter data set. In both instances the ‘base case’ model was used, i.e., the models that were used to set the precautionary yields for each fishery in 2009. The change in the biomass estimated by the models arising from withholding data was found to be highest in the earlier years of stock assessments, when each extra year of withheld data represented a large contribution to the model. In later years, the change in estimated biomass arising from withholding data was much smaller, with values generally within the 95% percentile of the MCMC run from the 2009 assessment. For the 2009 assessment removing up to four years of the early tag data had little effect on the estimate of the initial biomass, or the range of the estimates.
Running updated assessments with catch data, or with catch data and age frequencies, provided no further information on the biomass trajectories in the model considered. Tag recapture data provided the most information to the model, although the model results were not very sensitive to changes in years of tag release and recapture data available to the model. For example even when the number of tag recaptures in the most recent year was doubled or halved, the corresponding estimate of current biomass changed by less than 10%; furthermore, these estimates remained within the 95% percentile envelope estimated by the MCMC posteriors.
 

The exploratory fishery for Dissostichus spp. has now been operating for 14 years in Subarea 88.1 and for nine years in Subarea 88.2. Associated research has led to an assessment of the fisheries potential yield, but a source of ongoing uncertainty in the stock assessment concerns recruitment dynamics. Although pre-recruit Antarctic toothfish fish are caught in various locations during fishing operations, different vessels catch different length classes in different locations and in different years, making it impossible to develop a consistent time series that would monitor trends in recruitment over time from the fishery. At its 2010 meeting, the Scientific Committee agreed that a time series of recruitments from a well designed survey could be a useful input into a stock assessment model and requested members develop a pre-recruit survey design. This paper develops a proposal for a CCAMLR sponsored research survey in response to the Scientific Committee request.
We consider here the objectives of the survey, the target fish age and length classes to be monitored, appropriate survey strata size and location, the number of sets in each stratum required to achieve target c.v.s, and associated sampling methodologies. Further, we propose that the first survey be a pilot, with two main objectives:  

(i)    To establish the feasibility of developing a time series of longline surveys to monitor 80–100 cm TL toothfish in parts of SSRUs 881.J and 881.L in the southern Ross Sea using standardised gear in a standardised manner; and
(ii)    To carry out experimental depth stratified fishing in 500–600 m depth adjacent to the survey boundaries to establish the most appropriate depth strata for future surveys.

We are submitting this proposal to WG-SAM for further evaluation as requested by the Scientific Committee (SC-CAMLR-XXIX, para 3.185). We are particularly interested in ways to improve the survey design, as well as additional work that could be undertaken to demonstrate the use of the results of such a survey in the provision of advice to the Scientific Committee.
 

The potential for data poor exploratory fisheries in the CAMLR Convention area to provide sufficient data to lead to formal stock assessments of Dissostichus spp. is evaluated and recommendations are made for optimizing this potential.  The types of data that have led to successful assessments in the past are reviewed. Exploratory fisheries and research questions for subareas without currently assessed stocks (48.6, 58.4, parts of 88.1 and 88.2) are summarized. The multi-nation, multi-year exploratory fishery in the Ross Sea leading to an assessment of D. mawsoni and the fishery for D. eleginoides in the northern South Sandwich Islands are used as examples of what can be achieved using exploratory fisheries to provide information for stock assessment. Three sources of data potentially collectable from exploratory fisheries have proved successful in the past: 1) tag mark and recapture; 2) collection and reading of otoliths; and 3) recording of length, sex and gonad stage. It is suggested that research standards for data collected by vessels in exploratory fisheries be focused on providing these three types of data and that detailed descriptions of CPUE data be de-emphasized. Focusing effort on subareas/divisions in which tagged fish have already been released is recommended to maximize recapture potential for existing tags while increasing the numbers of newly tagged fish.  The subareas/divisions should be prioritized according to the amount of information currently available, and the prospect of acquiring a sufficient amount of data with which to conduct a formal assessment.

This paper presents the updates to the CCAMLR tagging database in relation to tag recapture links, and the problems associated with linking tags.