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Ecology of the Arrow Squid (Nototodarus gouldi) in Southeastern Australian Waters: A Multi-Scale Investigation of Spatial and Temporal Variability.

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Stark, KE (2008) Ecology of the Arrow Squid (Nototodarus gouldi) in Southeastern Australian Waters: A Multi-Scale Investigation of Spatial and Temporal Variability. PhD thesis, University of Tasmania.

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Abstract

Phenotypic plasticity together with a short lifespan, rapid growth and the ability to move over considerable distances mean that squid populations are extremely responsive to changing environmental conditions, and this generates highly variable and complex patterns of population ecology. This study examined the population ecology of the ommastrephid squid Nototodarus gouldi in southern Australian waters; investigating how patterns of distribution and abundance vary in space and time, and the factors that may be driving these patterns. Broad scale GIS and statistical (GAM and GLM) analyses of trawl fishery observer data identified clear ‘hotspots’ of N. gouldi abundance in southeastern Australian waters, which corresponded with areas of significant mesoscale oceanographic activity (i.e. strong shelf break fronts, convergence zones and upwelling). Abundance was seasonal, but this seasonality varied between locations. Remotely sensed sea surface temperature and chlorophyll-a concentration were unable to account for the spatio-temporal patterns in N. gouldi encounter and catch rates, possibly due to a temporal mismatch between local oceanographic activity and the evidence of its effect on N. gouldi. Lagged relationships between local environmental conditions and N. gouldi abundance were then examined on an annual scale for one region- the Bonney Coast, a ‘hotspot’ area subject to seasonal upwelling activity. Local wind speed and ENSO were both strongly correlated with annual abundance, most likely due to their influence on mixing and upwelling activity, and thus prey availability. Cross validation of a linear model incorporating these environmental variables suggested reasonably good predictive ability. A negative correlation between jig and trawl derived indices of abundance however suggests that inter-annual variability is driven by distributional changes as well as recruitment variability, with the depth distribution of N. gouldi possibly changing in response to the position of the upwelling front. Population structure and life history characteristics of N. gouldi on an inshore jig ground in southeastern Tasmania exhibited significant variability over four years, although patterns were not always consistent for the two sexes, particularly in relative levels of reproductive investment. Abundance was also highly variable over this four year period however there was no clear relationship between biological characteristics and available abundance. Squid in a year of extremely high abundance were a similar size and age to those sampled in years of low abundance; the change in biomass therefore attributed to changes in numbers of squid. Nototodarus gouldi appear to undertake ontogenetic bathymetric migrations, with squid recruiting to the jig fishery as small juveniles, growing and maturing over the summer before moving away, most likely into deeper waters where large mature individuals are caught by trawlers. A change in sex ratio over the jig season also indicates that males may leave the jig grounds earlier than females. Tracking of N. gouldi using an automated acoustic telemetry array also showed that squid moved away from the inshore jig grounds, but this movement was not in any way synchronous, with individualseason of the tracking study- a year of very low abundance. In contrast, the persistence of large mature N. gouldi on the jig ground when abundance was extremely high suggests the increased numbers of squid may be due to longer residency times and the accumulation of individuals. Thus, the ecology of N. gouldi, like many other commercially exploited ommastrephid squid, appears to be closely linked to hydrography and ocean productivity. They are in greatest abundance where the shelf break is strongly defined or where other mesoscale oceanographic activity is present (e.g. upwelling), and variability in biomass cycles also appears to be related to the seasonality and nature of local mesoscale oceanography. Further investigations are needed to elucidate the finer-scale variability and detail of the mechanisms driving these patterns. In particular, investigation into the linkages between populations on jig and trawl fishery grounds and the relationship between ontogeny and depth distribution appear to be critical for understanding patterns of distribution and abundance, and for the development of appropriate fishery assessment models.s apparently leaving over an extended time period in the season of the tracking study-a year of very low abundance. In contrast, the persistence of large mature N. gouldi on the jig ground when abundance was extremely high suggests the increased numbers of squid may be due to longer residency times and the accumulation of individuals. Thus, the ecology of N. gouldi, like many other commercially exploited ommastrephid squid, appears to be closely linked to hydrography and ocean productivity. They are in greatest abundance where the shelf break is strongly defined or where other mesoscale oceanographic activity is present (e.g. upwelling), and variability in biomass cycles also appears to be related to the seasonality and nature of local mesoscale oceanography. Further investigations are needed to elucidate the finer-scale variability and detail of the mechanisms driving these patterns. In particular, investigation into the linkages between populations on jig and trawl fishery grounds and the relationship between ontogeny and depth distribution appear to be critical for understanding patterns of distribution and abundance, and for the development of appropriate fishery assessment models.

Item Type: Thesis (PhD)
Keywords: squid populations, population ecology, ommastrephid squid, Nototodarus gouldi, southern Australian waters, catch rates
Date Deposited: 10 Nov 2008 23:04
Last Modified: 18 Nov 2014 03:52
URI: http://eprints.utas.edu.au/id/eprint/7905
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