Spatial structuring and patterns of connectivity among Antarctic toothfish (Dissostichus mawsoni) stocks in the Southern Ocean: a view through otolith chemistry
An important prerequisite of sustainable fisheries management is knowledge about the spatial structure of fish populations. Such information provides a basis for understanding population dynamics and connectivity as well as posing questions around a speciesˈ resilience to ongoing fishing pressure. For Antarctic toothfish (Dissostichus mawsoni), a bentho-pelagic fish species with a spatial distribution that encompasses much of the Southern Ocean south of about (60°S), aspects around population structure and connectivity are still uncertain. The basis of this study was to gain a better understanding of Antarctic toothfish population structuring across spatially discrete fishing areas located around the Antarctic continent. The primary aims were therefore to determine whether patterns of connectivity between these areas were evident and whether source or sink areas for toothfish existed across its Southern Ocean distribution which would be a key understanding towards effective management of toothfish populations. To that end, this study used fish otoliths (ear bones) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) to determine life history aspects of Antarctic toothfish. The first research chapter (Chapter 2) tests the efficacy of otolith microchemistry techniques by determining if otolith edge chemistry (corresponding to recent capture) could distinguish toothfish fishery grounds. Fish otoliths were obtained by scientific observers on board longline vessels operating across spatially discrete fishing areas in the Ross Sea (RS), Amundsen Sea (AMS), Southern Atlantic Ocean (SAO) and Southern Indian Ocean (SIO). Based on four elements (Al, Mg, Ba and Sr), significant spatial heterogeneity was shown among most regions indicating the water masses were quite different. The strongest patterns of separation were between the RS and SAO where significantly lower Sr compositions in the Ross Sea corresponded with a lower salinity water regime consistent with large scale freshening events within the Ross Sea. Conversely, Al. compositions were significantly lower in toothfish from the SAO (Tukey’s HSD, P < 0.001) than in the Ross Sea, consistent with fish exposure to the Al-depleted source waters in the Weddell Sea, whereas differences in Mg were possibly reflective of physiological effects linked to recent spawning in these regions. Spatial heterogeneity was further evident through quadratic discriminant function analyses (QDFA) with jack-knife classification rates for the Ross Sea (81%) and Southern Atlantic Ocean (79%). However, areas adjacent to the RS and SAO did not show sufficient otolith elemental differences to separate them, possibly the result of regional scale gyres and the Antarctic Circumpolar Current (ACC) mixing the water mass within and across adjoined areas. These findings suggest otolith chemistry can discriminate Antarctic toothfish populations across several spatially discrete fishing grounds throughout the Southern Ocean. It also has the potential to aid in future investigations aimed at discerning the spatial structure of toothfish stocks throughout the Southern Ocean and the extent to which stocks may be connected. In the second research chapter (Chapter 3), the spatial structure of Antarctic toothfish populations was examined from the otolith nuclei of toothfish collected from the Ross Sea (RS) Amundsen Sea (AMS) Southern Atlantic Ocean (SAO) and Southern Indian Ocean (SIO). ANOVA trace element analyses of the otolith nuclei of 10-year and 14-year-old toothfish indicated the water mass during early growth was quite different at least between the RS and SAO. That the age of fish was the same among group analyses limited any potential for overlapping noise in elemental tags between fish that may have spawned in different years (Elsdon et al 2008).Spatial heterogeneity of Antarctic toothfish during early growth was consistent with otolith edge compositions where low Al reflected fish exposure to Al-depleted source waters in the Weddell Sea and Mg which was higher in the closely adjoined RS and AMS compared to the SAO and SIO. However, spatial visualisations of the otolith nucleus compositions and agglomerative hierarchical cluster (AHC) analyses using Al, Mg, Sr and Li revealed separation between the RS and SAO that was not as clearly defined as in the otolith edge chemistry of the same fish (Chapter 2). Some individuals, primarily from adjoining areas downstream of the RS (AMS) and SAO, showed patterns of connectivity consistent with transport of larvae from upstream of these areas through the Antarctic Circumpolar Current. This would diminish any unique elemental signatures between areas similar to patterns observed in the otolith edge chemistry of the same fish and suggest the population structure of Antarctic toothfish is more spatially complex than the spatial management units of the fishery. Nonetheless, the indication of spatial heterogeneity between the RS and SAO was still evident suggesting larvae from these areas had close affinities to their respective capture locations highlighting these regions as important source areas for Antarctic toothfish. Overall, these findings provide supporting evidence to the existence of separate Antarctic toothfish populations between the Ross Sea and Southern Atlantic that is supplementary to genetic evidence between these regions. This will provide stronger grounds for fisheries management decisions for Antarctic toothfish stocks within these areas and throughout the Southern Ocean. In the third study (Chapter 4) otolith natal chemistries of Antarctic toothfish were evaluated to determine whether fish from two small scale research units (SSRU) 88.1C in the Ross Sea and 88.2H in the Amundsen Sea fishery could be differentiated by the trace elemental concentrations in the otolith edge. As a follow on to this, the potential of otolith nucleus signatures to distinguish stocks among these regions was also investigated. For the elements Mg, Al and Sr, patterns of spatial heterogeneity in otolith edge chemistry was shown. This resulted in the correct classification of 63% of fish overall to their original capture sites. However, discrimination of otolith natal signatures for adult age 17 toothfish using Al and Zn showed greater classification success (79%), compared to the otolith edge, which suggested that the majority of toothfish from SSRU 88.1C and 88.2H show similar patterns of structuring consistent with their known capture location, indicating they may have used different spawning habitats between areas. That adult toothfish in these analyses were of the same age, and likely subject to the same environmental conditions during early growth reduced the influence of indifferent chemistries associated with fish that may have been spawned in different years (Elsdon et al 2008). However, given these analyses only included adult toothfish, further investigations using a larger sample base of both adults and juveniles collected from shelf and slope regions within Ross Sea and Amundsen Sea would provide stronger evidence of structuring between these regions. In the final research chapter (Chapter 5), life history chronologies of Antarctic toothfish were examined from fish otoliths obtained in consecutive seasons (2012 –2013) from two longline operations within the Ross Sea. Specific chronologies were acquired using laser ablation ICP-MS to determine whether spatial variability in otolith chemistry could differentiate capture location and population structure of Antarctic toothfish in consecutive years. The otolith edge chemistries of adult Antarctic toothfish showed no significant spatial heterogeneity between the Pacific Antarctic Ridge (PAR) and continental slope in either season. The lack of spatial heterogeneity in the otolith edge chemistry of the same adult age classes contributed to the low discriminatory power and overall classification success between these areas in 2012 (47%) and 2013 (67%). This indicates the environmental conditions within the Ross Sea may be similar between years in these areas. This may have been due to regional scale hydrographic features driving the water mass from the PAR over the Slope and onto the continental Shelf (Locarnini 1994; Budillon et al. 2003) diminishing any distinguishable elemental profiles within these areas and in the otoliths of toothfish. Nevertheless, despite a lack of spatial heterogeneity among adult otolith edge compositions, significant spatial differences were strongly evident at least among the same subadult age classes in otoliths from 2012. This underscores patterns of variability in environmental conditions between years and age classes for toothfish in the Ross Sea. However, significant spatial heterogeneity in otolith edge chemistry between Slope and Shelf areas for Mg and Ba did not correspond with higher discriminatory power with classification success rates overall in 2012 (47%) and 2013 (55%) further evidence that Slope and Shelf areas are reasonably homogeneous with some seasonal variability. Such a finding will confound the ability to identify the contribution of recruits in one year from adults of unknown provenance in another year (Gillanders 2002) promoting data misinterpretation (Reis-Santos et al. 2012). This finding would indicate the use of otolith nucleus signatures in assessing connectivity of Antarctic toothfish over different years would be untenable if the same habitat markers associated with spawning areas vary over time. However, smaller sample sizes among 2013 collections may have contributed to the lack of statistical power and further analyses across a broader array of elements is perhaps recommended. Similarly, although fishing operations across small-scale research units (SSRU) were visited in both years, the same fishing areas themselves were not. This may have diminished more representative sampling comparisons. Also, the otolith nucleus chemistry of adult and subadult toothfish within the Ross Sea showed no significant spatial variability among elements (Al, Mg, Sr and Ba) in either year. This indicated larval growth among both toothfish age classes was much the same within the Ross Sea. Similarly, nonmetric multidimensional scaling visualisations showed no clear separation among adults suggesting that the majority of Antarctic toothfish from the Ross Sea fishery use a common spawning ground. However, one individual from the Slope in 2012 and two from the PAR in 2013 had nMDS distance measures outside the 95% confidence ellipses suggesting these fish may have utilised different spawning grounds. While these findings support the notion of a single spawning population for Antarctic toothfish within the Ross Sea over consecutive years, the lack of representative sample sizes in 2013, along with the inability to sample the same locations in each year (due to fishing restrictions or variable sea ice conditions) poses some uncertainty around elemental tags, and further investigation is required. Nevertheless, the findings are in line with genetic evidence and similar otolith microchemistry approaches (Smith & Gaffney 2005; Hanchet & Rickard 2008; Kuhn & Gaffney 2008; Ashford et al. 2012). A small proportion of individuals had otolith nucleus chemistries that suggest they utilised different spawning areas outside of the Ross Sea indicating the population structure of Antarctic toothfish is more spatially complex than the more confined fisheries management areas of the Ross Sea.
Otolith microchemistry, Antarctic toothfish, Population structure, Life history, Southern Ocean
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