Candy, S.G. (2011), Estimation of natural mortality using catch-at-age and aged mark-recapture data: a multi-cohort simulation study comparing estimation for a model based on the Baranov equations versus a new mortality equation.

An estimation strategy for natural mortality, M, using multiple cohorts and multiple years of catch-at-age and aged mark-recapture data was tested using simulation. Alternative fishing selectivity functions of age of dome-shaped versus sigmoidally shaped were applied. Two alternative estimation models were developed both using a Poisson likelihood for annual number of recaptures-at-age and model the population numbersat- age by annual difference equations obtained by integrating an ordinary differential equation (ODE) for within-year population dynamics. The 'fully parametric' BODE model is based on the Baranov ODE while the 'semi-parametric' constant catch ODE (CCODE) model uses a new total mortality ODE with constant within-year catch per unit time and does not estimate annual fishing mortality rates (i.e. the F’s) or fishing selectivity function parameters. It removes the actual, considered known, catch-at-age numbers directly from the population. Estimation for the BODE model requires an extra component to the log-likelihood which defines the process error in predicted catch-at-age numbers.

Simulations assumed 1 000 releases per year over 12 years with recruitment average of 2 million with annual coefficient of variation (CV) of 0.3 and annual catch of 500 000. Simulations which passed catch-at-age numbers to the estimation algorithm after perturbation by observational error were also carried out for each model in order to investigate the effect on estimation of M. Simulations carried out without observational error showed that when all parameters were jointly estimated and selectivity was domeshaped, estimation of M was unreliable for both models but more so for the BODE model. The reason for this is explained by the confounding of selectivity parameter estimates with that for M. In contrast, when sigmoidally shaped selectivity was simulated, and the functional form of selectivity was correctly specified in the BODE model, both models gave close to unbiased and reasonably precise (CVs of 0.07 to 0.14) estimates of M, but the BODE model estimate was substantially more precise. However, when a minor misspecification of the functional form of selectivity was fitted by the BODE model, in comparison the CCODE model gave superior accuracy. When realistic observational error in catch-at-age numbers was included in simulations and combined with the sigmoidally shaped selectivity function, the bias and imprecision in estimates of M increased by no more than 2% for the CCODE model with no increase detectable for the BODE model. With these caveats, both models can be used to estimate this notoriously difficult parameter with the profile likelihood a useful indicator of the degree of success of estimation, even if some bias remains.

Candy, S.G., D.C. Welsford, T. Lamb, J.J. Verdouw and J.J. Hutchins (2011), Estimation of natural mortality for the Patagonian toothfish at Heard and McDonald Islands using catch-at-age and aged mark-recapture data from the main trawl ground

Attempts to estimate natural mortality, as a single constant M, simultaneously with other model parameters in integrated assessments via CASAL for the Heard and McDonald Islands (HIMI, Division 58.5.2) Patagonian toothfish (Dissostichus eleginoides) fishery have been unsuccessful. An alternative strategy was adopted whereby the relatively long time series of catch-at-age and mark-recapture data from the main trawl ground at HIMI were used to estimate M. Catch and releases by age class for this fishery were obtained using proportions-at-length and fishery- and year-specific age–length keys (ALKs) for years 1998 to 2008. A large proportion of the recaptures of fish released in this fishery were aged and these were used to obtain recapture numbers by age class. Two alternative estimation models were programmed in R, based on alternative ordinary differential equations (ODE) for within-year population dynamics. These are the BODE model (Baranov ODE) and the CCODE model (constant catch ODE). The CCODE model is a new model for describing total mortality which disaggregates fishing and natural mortality differently to the Baranov equations and does not require a catch equation but removes catch-at-age numbers directly from the estimates of population numbers-at-age. The properties of these two models for estimation of M have been studied using simulation. In application to the data obtained for the HIMI main trawl fishery, the CCODE model gave a well-behaved profile for the log-likelihood with the corresponding estimate of M of 0.155, however, the 95% confidence bounds of the estimate were very wide ranging from 0.055 to 0.250 (based on a Poisson over-dispersion estimate of 3). In contrast, the BODE model gave unrealistic estimates of M and the annual fishing mortality rates.

Welsford, D.C (2011), Evaluating the impact of multi-year research catch limits on overfished toothfish populations

In stocks that have been depleted by overfishing, the benefit of additional fisheries removals for research purposes needs to be evaluated against the risk that such catches may contribute to delaying or preventing the recovery of such stocks. Through simulating a Patagonian toothfish (Dissostichus eleginoides) stock that has been subjected to varying levels of overfishing and research catches, this study shows that stocks can take decades to recover even in the absence of fishing, and relatively low levels of research catch can significantly delay the recovery of a stock. Scenarios assessed in this study indicate that, where a stock has been depleted to at or below 20% of the median unfished spawning stock biomass, research catches in excess of 0.6% of median unfished total stock biomass should be avoided to ensure that research does not significantly impact on the recovery of depleted stocks in the long term. Research catches may need to be even lower where there is uncertainty regarding the key parameters related to stock productivity, such as growth, maturity, recruitment or natural mortality rates.

Ziegler, P.E., D.C. Welsford and A.J. Constable (2011), Length-based assessments revisited – why stock status and fishing mortality of long-lived species such as toothfish cannot be inferred from length-frequency data alone

In many of CCAMLR’s new and exploratory fisheries, stock assessment techniques are still in development. The failure of the tag-recapture program to provide robust assessments in many areas and doubts about using unstandardised catch rate data have led to a desire to draw inferences about stock status and fishing mortality from length-frequency data alone. In this paper, a number of length-based stock assessment methods are revisited and some limitations in their ability to draw robust inferences about stock status and fishing mortality for long-lived fish species such as Patagonian toothfish (Dissostichus eleginoides) are illustrated. While length-frequency data can be useful in assessments when integrated with other informative estimators for stock abundance and productivity, the review and the simulations highlight that interpretation of raw length-frequency data and single length-based indicators for stock status and fishing mortality is problematic and should be avoided.

Sutton, C.P. and P.L. Horn (2011), A preliminary assessment of age and growth of Antarctic silverfish (Pleuragramma antarcticum) in the Ross Sea, Antarctica

Antarctic silverfish (Pleuragramma antarcticum) were sampled during a trawl survey in the Ross Sea, Antarctica. Biological data, including fish length, weight, sex, gonad maturity, liver weight and diet analysis were collected from 311 specimens. Standard length and weight were well correlated (r² = 0.99).

Counts of growth zones in 304 thin-sectioned otoliths were used to estimate ages and von Bertalanffy growth parameters. The species is relatively slow-growing with a moderate longevity; the maximum estimated age was 14.3 years. Von Bertalanffy parameters derived for both sexes combined are: L_∞ 22.1 cm SL; K 0.167 y^–1; t₀ –0.4 years. Parameter estimates were also derived for the sexes separately. Female Antarctic silverfish appear to reach a larger size than males, but none of the estimated von Bertalanffy parameters were statistically significantly different between sexes. All parameter estimates are preliminary as the ageing method is unvalidated and about two-thirds of the sampled fish could not be sexed.

Precision estimates and age bias plots indicated that there was good within-reader and between-reader agreement, so the otolith sections appear able to be consistently interpreted.

The standard lengths of the sampled Antarctic silverfish ranged from 4.6 to 22.9 cm. Pronounced modes in the length-frequency distribution occurred at 7.1–7.5, 10.6–11.0 and 15.1–15.5 cm. Mean lengths of 4.9 and 7.3 cm (at ages 1.3 and 2.3 years respectively) are consistent with those presented in the literature. The age-frequency distribution exhibited a mode from age 6 to 9 years.

Parker, S.J. and M.H. Smith (2011), Can toothfish catches be used to predict the presence of vulnerable benthic invertebrate taxa?

Accurate estimation of the impact of bottom fishing on vulnerable marine ecosystems (VMEs) requires knowledge of the distribution of those communities relative to the fishing footprint. If high target species catch rates are associated with habitats where VMEs are found, impacts from fishing would be higher than if VMEs are distributed randomly with respect to fishing locations. This study used the catch of the six most common vulnerable invertebrate taxa reported by observers on New Zealand vessels during the 2009/10 Ross Sea longline fisheries to correlate toothfish catch rates and benthic invertebrate catch rates at the scale of a longline segment, ~1 200 m. Analysis of the data available showed no evidence that the presence of any of six VME indicator taxa was informative in predicting Antarctic toothfish (Dissostichus mawsoni) catch at the scale of a longline segment. This supports conclusions of previous work finding no relationship between aggregate VME indicator taxa weight and toothfish catch at the scale of a longline set, ~7 km. Further studies at intermediate scales (10–100 km) would be useful to determine if both toothfish and individual VME indicator taxa have regionally concentrated distributions, showing a high degree of spatial overlap with the fishery.

Calise, L. and G. Skaret (2011), Sensitivity investigation of the SDWBA Antarctic krill target strength model to fatness, material contrast and orientation

The distorted-wave Born approximation model is recognised as state-of-the-art in predicting acoustic target strength (TS) from fluid-like marine organisms. A stochastic version, the so-called SDWBA model, is endorsed by CCAMLR to predict the TS of Antarctic krill (Euphausia superba) for use in the acoustic echo-integration method. In this study, the SDWBA TS pattern and frequency response behaviours at standard survey frequencies and animal lengths are explored via a sensitivity analysis of some important parameters: number of stochastic realisations, the fatness factor, the material contrasts between the organism and the surrounding seawater and the distribution of animal orientations. Due to the high variability of TS to realistic changes in parameters, it is important that the model should be parameterised according to the potential seasonal variability of krill in the area of investigation. In a multifrequency context, the girth–length relationship for the body shape parameterisation can give a high source of uncertainty, and there is an urgent need for development of accurate and standard procedures to measure both the material property contrasts and the orientation at the time and location of the actual survey. Moreover, a number of incongruities within the CCAMLR SDWBA model package are identified and proposed solutions implemented in an improved package.

Sologub, D.O. and A.V. Remeslo (2011), Distribution and size–age composition of Antarctic krill (Euphausia superba) in the South Orkney Islands region (CCAMLR Subarea 48.2).

The distribution, size–age composition and biological condition of Antarctic krill (Euphausia superba) caught by the Russian krill trawler FV Maxim Starostin in Subarea 48.2 between January and March in 2009 and 2010 are described. The distribution of size–age groups in association with particular water masses indicated that age 2+ krill dominated in cold Weddell Sea waters in the southern parts of the fishing area, while older age classes (3+, 4+, 5+) dominated in Antarctic Circumpolar Current (ACC) waters in the northern parts. No 1+ age cohort was found in catches in 2009 or in 2010. This absence of 1+ E. superba is unlikely to be a result of trawl selectivity as adult Pygmy krill (E. frigida) (of the same size as 1+ krill) were present in the catches.

Foster, J., S. Nicol and S. Kawaguchi (2011), The use of patent databases to predict trends in the krill fishery

The fishery for Antarctic krill (Euphausia superba) caught 210 000 tonnes in 2009/10 with vessels from six of CCAMLR’s 25 Members participating in the fishery. Trends in the krill fishery are currently informed by data on annual catches and by CCAMLR’s annual notification procedure. There has been considerable discussion within the Scientific Committee over the utility of these sources of information to detect trends in the fishery, and of the need to obtain more information from the krill industry so that developments that affect future catches can be predicted. This paper uses publicly available patent databases to examine both the number and type of patents related to krill that are lodged annually. This information shows similar trends to the catch and notification information, and the three sources of information can be used in combination to provide information to the Commission on potential changes in the krill fishery. The Scientific Committee has recommended that the patent database be maintained by the Secretariat so that it can annually update a register of krill-related patents which will be of use for indicating potential future trends in the krill fishery.

Taki, K., M. Kiyota, T. Ichii and T. Iwami (2011), Distribution and population structure of Dissostichus eleginoides and D. mawsoni on BANZARE Bank (CCAMLR Division 58.4.3b), Indian Ocean.

The distribution, size composition, sex ratio and sexual maturity of Dissostichus eleginoides and D. mawsoni from BANZARE Bank were investigated using biological data collected by a Japanese commercial fishing vessel during the austral summer fishing seasons (December–February) from 2006/07 to 2008/09. Dissostichus eleginoides was mainly caught on shallower banks (<1 000 m), while D. mawsoni was caught on deeper slopes (>1 500 m). Separation of the two species by depth may be related to physical intolerance to cooler temperatures because of the lack of antifreeze in the former species. Dissostichus eleginoides showed a resting stage in sexual maturity, while D. mawsoni was in developing condition. In D. mawsoni, small fish contributed a very low proportion to abundance, suggesting that no substantial recruitment takes place on BANZARE Bank and the populations consist primarily of adults migrating from adjacent Antarctic coastal areas. The life cycle of D. mawsoni on BANZARE Bank is similar to that hypothesised for the Ross Sea population with analogous latitudinal patterns in sexual maturity and size composition. The study indicates that the management of Dissostichus spp. on BANZARE Bank and adjacent waters needs to consider species-specific distribution and possible migration.

Iwami, T., M. Naganobu, K. Taki and M. Kiyota (2011), Annual changes in species composition and abundance of myctophid fish in the north of South Georgia (Subarea 48.3), Antarctica, during austral winters from 2002 to 2008

Species composition and abundance of myctophid fish were analysed using the fish by-catch samples collected by scientific observers on board Japanese commercial krill fishing vessels in the area north and northwest of South Georgia, Antarctica, during austral winters from 2002 to 2008. A total of 10 myctophid species were identified in the by-catch samples, among which Krefftichthys anderssoni, Gymnoscopelus nicholsi and Protomyctophum choriodon were abundant. Different life stages of K. anderssoni and G. nicholsi from larvae to adults were found, suggesting that these species may have their nursery ground around South Georgia. Electrona antarctica was not a major component of the recent mesopelagic ichthyofauna in this area, although this species used to be dominant in the Antarctic coastal waters. In contrast, P. choriodon, which is known as a south temperate species, became dominant in recent samples. Additionally, the size distribution of P. choriodon was unimodal, indicating that individuals in the samples belonged to the same size group and probably migrated from the population in warmer northern areas.

Editors (2011), Editorial

This is the 18th volume of CCAMLR Science and the contents reflect the important science that supports CCAMLR’s management of the marine living resources of the Southern Ocean. In particular there is a suite of papers that address key aspects of improving assessment of the population status of toothfish and of assessing the risk associated with gathering data and interpreting indicators from populations that may be depleted. The suite of papers on krill cover the spectrum of analysis of data gathered on fishing vessels to using trends in patents to predict the future for krill fishing; as well as a description of the analysis of acoustic data to determine krill biomass.

During 2010 there were five meetings of the specialist groups of the Scientific Committee of CCAMLR, covering acoustics, statistics and modelling, ecosystem monitoring and management, fish stock assessment and at-sea operations. A total of 104 working papers were presented at these meetings. This volume consists of papers that were submitted to those meetings and have been through the scrutiny of those specialist groups as well as the more traditional peer-review process.

The papers in this volume of CCAMLR Science broadly reflect the important areas of research used to provide advice on the management of fishing in the Convention Area. For example, in the case of toothfish there are papers that develop our understanding of the dynamics of fish populations through improved natural mortality estimates, examine the potential costs of research catches as well as the potential spatial overlap with vulnerable benthic habitats. In the case of Antarctic krill, there are papers that describe how abundance estimates are derived from acoustic data, details of the krill and fish by-catch taken in the krill fishery and a novel paper looking at market predictors of the future krill fishery.

Publication of this volume of CCAMLR Science coincides with the 30th meeting of the CCAMLR Scientific Committee. Over those 30 years, several thousand scientific papers have been presented to CCAMLR meetings and provide the basis for the science-based decision-making by CCAMLR. One of the most striking changes over the past 30 years is the increase in the number of scientific journals in which work of relevance to CCAMLR is published. This is undoubtedly driven by the need to publish (or be damned), however, quite often authors refer to their findings being ‘important to CCAMLR’ yet those papers do not get submitted for consideration by CCAMLR. I would urge all authors who consider that their papers may be relevant to CCAMLR to contact the CCAMLR Representative for their country (or the CCAMLR Secretariat) to investigate ways to maximise the potential impact of their important science.