CCAMLR Science, Volume 17 (2010)
This is the 17th volume of CCAMLR Science and the contents reflect the diverse science that supports CCAMLR’s management of the marine living resources of the Southern Ocean. There is a truly circumpolar feel to this volume with papers using data from the Pacific, Indian and Atlantic sectors of the CAMLR Convention Area. The five papers concerning the Ross Sea further contribute to our knowledge of a fascinating part of the Southern Ocean and demonstrate the important role that data from fisheries can provide in contributing to a better understanding of the ecosystem dynamics of the region.
During 2009 there were a total of eight weeks of meetings of the specialist groups of the Scientific Committee of CCAMLR, covering fisheries acoustics, statistics and modelling, ecosystem monitoring and management, vulnerable marine ecosystems, incidental mortality associated with fishing, fish stock assessment and at-sea operations. A total of 167 working papers were presented at these meetings. This volume consists of papers submitted to each of those working groups, papers that have been through the scrutiny of those groups as well as the more traditional peer-review process.
The breadth of information considered by CCAMLR is central to providing the best-available science upon which to base decisions concerning the management of fisheries in the Southern Ocean. It also reflects a great deal of time and energy of a relatively small group of scientist. The need to identify ways to share the burden of delivering scientific input, and to help build the capacity to deliver excellent science into the future, has been recognised by CCAMLR through the establishment of a General Science Capacity Fund. Through this fund, CCAMLR will support scientific development, particularly through engaging earlycareer scientists, in the work of CCAMLR. The continued support for the excellent science submitted to CCAMLR, and published in CCAMLR Science, underpins the future sustainable management of the marine living resources of the Southern Ocean.
A quantitative food web of the Ross Sea is presented here as a step towards investigating ecosystem effects of the fishery for Antarctic toothfish (Dissostichus mawsoni). The model consolidates quantitative information on trophic links across all the major biota of the Ross Sea and tests for data consistency. The model has 38 trophic groups and is balanced in terms of annual flows of organic carbon in an average recent year (1990–2000). The focus of the model is on the role of Antarctic toothfish in the food web which means that the model has greater taxonomic resolution towards the top of the food web than the base. A survey of the available literature and both published and unpublished data provided an initial set of parameters describing the annual average abundance, imports, exports, energetics (growth, reproduction, consumption) and trophic linkages (diets, key predators) for each model group. The relative level of uncertainty on these parameters was also estimated. This set of parameters was not self consistent, and a method is described to adjust the initial parameter set to give a balanced model, taking into account the estimates of parameter uncertainty and the large range of magnitude (>6 orders of magnitude) in trophic flows between groups. Parameters for biomass, production rate, growth efficiency, diet fractions and other transfers of biomass between groups were adjusted simultaneously. It was found that changes to the initial set of parameters needed to obtain balance were reasonably small for most groups and most parameters. The mean absolute change for all key parameters (biomass, production rate, growth efficiency) and all groups together was 1.7%, and for diet fractions was 0.6%. Large but not implausible changes in biomass, production/biomass and production/consumption parameters were needed to balance the microzooplankton (34–47%), ice bacteria (61–72%), and ice protozoa (24–54%), components of the model. Trophic levels are in close agreement with those derived from isotope and other ecosystems. In the balanced model, there is only enough large (>100 cm) toothfish production to satisfy 6.5% of the diet of Weddell seals, 5.6% of the diet of orca and 2.6% of the diet of sperm whales. The model does not support the hypothesis that depletion of Antarctic toothfish by fishing would change the diet of predators of toothfish (Weddell seals, orca, sperm whales) by large amounts throughout the Ross Sea, though the importance of toothfish as prey items to these predators is not tested and requires further investigation. The model shows that large toothfish consume 61% of the annual production of medium-sized demersal fishes and 14% of the annual production of small demersal fishes, implying a potential for the fishery to affect these prey through trophic cascades. There is a need to establish monitoring of medium and small demersal fishes in the Ross Sea, and to model potential changes to these groups due to the fishery. The following documents have been made available supporting this paper:
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This paper summarises knowledge of the distribution and relative abundance of Antarctic toothfish (Dissostichus mawsoni) on the Ross Sea shelf (defined here as the continental shelf out to the edge of the shelf break at about 600–800 m depth). The focus is on D. mawsoni catches from the shelf, as this area is likely to have the greatest overlap between D. mawsoni and potential predators, and thus where any ecosystem effects would be most likely to occur. The shelf catch has been taken mainly in depths greater than 800 m from three localised fishing grounds of deep water off Terra Nova Bay, Ross Island, and in the south of small-scale research unit (SSRU) 881L (adjacent to the Ross Ice Shelf). The catch rates from the exploratory longline fishery typically show high temporal and spatial variability, even between consecutive sets within the main fishing grounds. Most toothfish caught in the southern Ross Sea were sub-adult and maturing fish, typically ranging from 60–130 cm in length, with some evidence for an ontogenetic migration from east to west as they grow. From the fisheries data available, there is no evidence for a northward contraction in the range of D. mawsoni over the course of the fishery. Nevertheless, it would seem prudent to have a monitoring system in place so that changes in relative abundance of these subadult fish could be detected. It is recommended that CCAMLR consider developing a sub-adult longline survey to monitor this part of the population. |
This study uses histological assessments to determine age- and length-at-spawning for female and male Antarctic toothfish (Dissostichus mawsoni) from fish sampled in the Ross Sea spanning the 2000–2009 fishing seasons. A characterisation of the oocyte developmental cycle of D. mawsoni shows that once development begins, oocytes grow and accumulate at the cortical alveoli stage for at least one year. Individual oocytes are then recruited into the vitellogenic phase over at least a 6–12 month period, resulting in a developed group of oocytes accumulating at the final maturation stage by approximately May each year. The age at 50% spawning for females on the Ross Sea slope region is 16.6 years (95% CI 16.0–17.3) or 133.2 cm (95% CI 130.9–135.7) by length. On average, |
Target strength (TS) of preserved (frozen and defrosted) Antarctic silverfish (Pleuragramma antarcticum) of total length (TL) from 110 to 202.5 mm were measured ex situ (Ancona Bay) at 38, 120 and 200 kHz in May 2007 and in February 2009. Overall ex situ TS–TL relationships were: TS38 = 36.83Log TL(cm) –103.62 (N = 18); TS120 = 34.75Log TL(cm) –84.20 (N = 19); TS200 = 26.71Log TL(cm) –73.74; (N = 16). Nine in situ acoustic trawl experiments targeted at juveniles and post-larvae of P. antarcticum with a total length from 13 to 70 mm, were conducted in the southeast of the continental slope and shelf of the Ross Sea at depths from 30 to 120 m, during the Italian expeditions of 1997/98, 2000 and 2004. The regression lines for estimating TS from nine experiments were: TS38 = 57.58Log TL(cm) –123.77; TS120 = 51.94Log TL(cm) –105.92; TS200 = 46.61Log TL(cm) –95.29. These results indicate that the TS–TL relationships at 38, 120 and 200 kHz of adults of P. antarcticum differ consistently from those of post-larvae and juveniles. However, a simple model derived from the geometrical and acoustic characteristics of different parts of adult P. antarcticum and designed to fit fish life stages from adults to post-larvae and frequencies from 38 to 200 kHz, can predict the experimental TS measured both ex situ and in situ with a mean difference <1.5 dB, although the differences between the predicted and measured TS at some fish lengths were high (from 7.16 to –3.14 dB). The model shows that the main contribution to the scattering for small fish is from soft tissues and for larger sizes from hard tissues (e.g. skeletal structures). This may explain why small and large fish have different TS–TL relationships. In the authors’ opinion the model could be used to give a first indication of the absolute abundance of P. antarcticum in the Ross Sea. A critical point is the decision rule to be used to differentiate P. antarcticum from Euphausia superba and E. crystallorophias that are present in the same region of the Ross Sea. Following the three-frequency decision criterion described in Azzali et al. (2004), some sizes of P. antarcticum could be confused acoustically with E. crystallorophias, but not with E. superba. |
Implementing measures to avoid significant adverse impacts to vulnerable marine ecosystems (VMEs) requires a specific list of the taxa that are considered vulnerable in the fishery being assessed. New Zealand identified an interim list of taxa to monitor in fishery by-catch in the Ross Sea as part of its benthic fisheries impact assessment. The rationale for including or excluding each taxonomic group is described relative to the group’s fragility, longevity, organism size and spatial distribution. Additional considerations such as organism mobility, community diversity, endemism, taxonomic resolution and presence in fishery by-catch are also important. Thirteen coarse-resolution taxonomic groups are identified as vulnerable to bottom longline fishing activities in the Ross Sea region, including two indicator-only taxa. |
The current CCAMLR assessment method for mackerel icefish (Champsocephalus gunnari) in Subarea 48.3 employs the CCAMLR mixture analysis program (CMIX) and generalised yield model (GYM) packages, which derives population numbers-at-age and projects these numbers forward under the given harvest control rule to set a two-year total allowable catch (TAC). One issue is the accurate identification of age cohorts from the survey data. In this paper a length-based approach is described that removes the problem of cohort determination. A bootstrap technique (consistent with the current CCAMLR approach) is used to estimate the length distribution of the population from the survey data which, in conjunction with the bootstrapped survey biomass data, yields an estimate of the population numbers-at-length. A comparison of this method and the existing age-based CMIX/GYM methodology showed very good agreement in calculated yields for 2006, 2007 and 2009, and some differences for 2008 which were explained by an unusual length distribution from which it was difficult to reliably estimate age composition in CMIX. Using the 2009 survey data and the new methodology, a TAC was calculated according to the CCAMLR decision rules of 1 577 and 933 tonnes for the 2009/10 and 2010/11 fishing seasons respectively. The respective estimates using the existing CMIX/GYM methodology were 1 590 and 968 tonnes respectively. |
Data collected as part of the CCAMLR Scheme of International Scientific Observation from conventional krill trawl vessels fishing in South Georgia (CCAMLR Subarea 48.3) were analysed using variance components analysis (VCA) to determine the relationship between observer coverage and our ability to reliably estimate parameters of interest, in this case the mean (or median) krill length and the rate of larval fish by-catch. A partial coverage sampling program has been implemented in South Georgia since 2002: observers have been placed on approximately 50% of vessels fishing in any one season, and have been present on board for about 30% of that season. They have achieved a rate of krill sampling equivalent to 18% of total hauls per vessel per season and 31% per vessel per season for larval fish. Between-vessel and between-haul variance was estimated. For krill mean length and larval fish by-catch rate, between-vessel variance (about 45% of total variance) was slightly lower than between-haul variance. However, the ratio is sufficiently close to 50% that sampling needs to be efficient at both vessel and haul level. It is proposed that an efficient sampling proportion, at least for South Georgia, should be >50% of vessels sampled each season, 20% of total season hauls sampled for krill and an equivalent or higher sampling proportion for larval fish. The Scientific Committee’s method of systematic partial coverage appears to have been sufficient in South Georgia to determine appropriate coverage levels in that fishery. The South Georgia data suggest that such strategies should be pursued for at least four years before the Scientific Committee will have sufficient data to determine appropriate sampling strategies in a particular area. |
In recent years there has been an increased focus on reducing seabird captures that occur during hauling in CCAMLR longline fisheries. Haul captures were first recognised by CCAMLR as a problem as early as 1994/95 when steps were taken to reduce the attraction of seabirds to vessels during the hauling process. Since 2003, increased efforts have been made to improve the design and performance of bird exclusion devices (BEDs) placed around the hauling bay. Data collected by scientific observers since 2003 suggest that there are two key aspects of effective BEDs, firstly that they provide a deterrent to seabirds landing adjacent to the line as it is being hauled, and secondly that seabirds are deterred from swimming or ‘jumping’ into the area around the hauling bay. Based on this analysis, CCAMLR incorporated these two key functional characteristics into the specification of a BED to be deployed by longline vessels to reduce seabird captures during the haul. |
Observer data collected on longliners between 2003 and 2009 were analysed to look at the levels of depredation caused by killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus) around South Georgia. Since 2003, cetaceans have been observed on 22% of 14 300 observed lines, with killer whales present on 3.8% and sperm whales on 17.7% of lines. Killer whales appear in pod sizes normally of 4 to 10 animals, and often appear to actively seek out fishing vessels and ‘strip’ the line of a large number of toothfish, usually depressing CPUE by about 50%. Sperm whales occur in smaller pod sizes, normally between 1 and 4 animals, and have a relatively lower impact on catches, depressing CPUE by up to 20%. Sperm whales have been more frequently encountered in recent years, occurring in larger pod sizes, whereas killer whale encounters and pod sizes have remained relatively constant. Most interactions from sperm whales occur during May at the start of the season with the sightings becoming fewer towards the end of the season in August. Killer whale interactions appear to be more consistent with no obvious pattern between months. Both species demonstrate an east to west migration throughout the season that is not related to the pattern of fishing effort. By comparing catch rates with and without the presence of cetaceans, accounting for other determinants of toothfish CPUE through a generalised linear model, it is estimated that the amounts of toothfish removed from longlines by cetaceans have varied between 1% and 8% of the declared catches over the period 2003–2009, with an average of 3.6%. |
Within the Crozet Islands Exclusive Economic Zone (EEZ), the Patagonian toothfish (Dissostichus eleginoides) longline fishery is exposed to high levels of depredation by killer (Orcinus orca) and sperm whales (Physeter macrocephalus). From 2003 to 2008, sperm whales alone, killer whales alone, and the two species co-occurring were observed on 32.6%, 18.6% and 23.4% respectively of the 4 289 hauled lines. It was estimated that a total of 571 tonnes (€4.8 million) of Patagonian toothfish were lost due to depredation by killer whales and both killer and sperm whales. Killer whales were found to be responsible for the largest part of this loss (>75%), while sperm whales had a lower impact (>25%). Photo-identification data revealed 35 killer whales belonging to four different pods were involved in 81.3% of the interactions. Significant variations of interaction rates with killer whales were detected between vessels suggesting the influence of operational factors on depredation. When killer whales were absent at the beginning of the line hauling process, short lines (<5 000 m) provided higher yield and were significantly less impacted by depredation than longer lines. Also, when facing depredation, it is recommended that vessels leave their fishing area and travel distances >40 n miles to prevent killer whales from finding them within a few hours. Although more data are still needed to better understand the way killer whales search and detect vessels, this study gives preliminary insights into possible mitigation solutions to the widespread depredation issue. |
The purpose of ecosystem monitoring programs is to indicate the state of ecosystems and whether they have been impacted by activities such as fishing. This paper discusses a range of methods for inferring such impacts using monitoring data with no control sites. These methods assess either (i) the expected probability of an observed value in an unimpacted system, or (ii) the frequency of values below a fixed reference point. The second approach allows inference criteria based on changes in this frequency rather than by reference to a critical probability. All methods would have provided a sustained indication of a significant decline in Antarctic fur seal (Arctocephalus gazella) pup production at South Georgia from the early 1990s within a few years of its onset, but a fixed reference point method could have provided this sustained indication from the onset. Furthermore, simulation of all methods suggests that the total probability of error (false positives and false negatives combined) is lowest with fixed reference point methods. The probabilities of Type I and Type II error can be evaluated analytically for these methods, which facilitates decision-making based on attitudes to risk. |
This work describes a parametric bootstrap model for standardising animal count data to a common reference point of breeding chronology for species with a complex temporal function of sampling availability. ICESCAPE (Integrating Count Effort by Seasonally Correcting Animal Population Estimates) is a suite of routines that implements a general abundance estimator accounting for availability bias, detection bias and sampling fractions less than unity. Within this resampling framework, all reported measures of uncertainty associated with originally published counts are propagated through to the final adjusted estimates. Adjustment for availability bias is achieved by applying an adjustment factor based on independently measured time series of availability throughout a breeding season. Such time series are typically collected at only a limited number of sites, so surrogate availability information for a site is used when none exists. Importantly, a common standardisation procedure allows site-specific estimates to be aggregated to achieve region-scale population estimates. By way of illustration, the method is applied to several examples of published studies of Adélie penguin abundance at breeding sites in Antarctica. These examples focus on adjusting counts of adults to an effective number of breeding pairs, although the software has been developed to accommodate adjustment and aggregation of other count objects typical for penguin species, such as occupied nest or chick counts. While tailored for Adélie penguins, the method and implementation is sufficiently general to be easily adapted for other colonial land-breeding species showing seasonal variation in availability to sampling methodology. |
New methods are presented for the collection and development of data to adjust counts of Adélie penguins made at any time in the breeding season to an estimate of the breeding population. The development of ‘availability adjustment factors’ involves the collection of time-series counts of population objects, such as adults, nests or chicks, throughout the breeding season and standardisation of the time series to a reference point one week after |
| Editors (2010), Editorial |