Age, growth and population dynamics of tropical squid and sepioid populations in waters off Townsville, North Queensland, Australia

Jackson, George David (1991) Age, growth and population dynamics of tropical squid and sepioid populations in waters off Townsville, North Queensland, Australia. PhD thesis, James Cook University.

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Abstract

Growth and ageing research was undertaken for squid and sepioid populations in waters off the Townsville region, North Queensland, Australia. The study species were the sepioid Idiosepius pygmaeus, and the loliginid squids Loliolus noctiluca, Loligo chinensis, Loligo sp. 1 and Sepioteuthis lessoniana. Age and growth parameters were determined by enumerating growth rings within the statolith microstructure of each of the species. Various mounting and rinding/polishing techniques were employed to reveal the growth rings. Daily periodicity in statolith growth rings was demonstrated to exist in 24 individuals in four of the study species; I. pygmaeus (6), L. noctiluca (9), L. chinensis (2) and S. lessoniana (7). Ring periodicity was determined by staining the statoliths in situ with tetracycline or calcein, and then maintaining the individuals in captivity to compare the rings laid down to the number of elapsed days. Double staining techniques were also employed to determine the rings laid down between stainings.

Field captured individuals of each of the study species were aged to construct growth curves. Growth was found to be rapid and maturity was found to be reached surprisingly fast. Tropical squid and sepioid growth is more correctly measured in days rather than months or years. The ageing studies with all five species did not reveal any individuals older than 200 days.

The ageing techniques developed were employed to discern any seasonality in squid and sepioid growth patterns. Regular sampling of I. pygmaeus and L. chinensis revealed different patterns of abundance, with I. pygmaeus more common during the winter period while L. chinensis was more abundant during the summer period.

Comparing growth based on statolith ageing to growth based on ELEFAN analysis on length frequency data revealed two diverging results. Length frequency analysis suggested that squid growth was asymptotic and that the life cycle was perennial. In contrast ageing research revealed that life span was short and that growth was rapid and non-asymptotic.

Seasonal size-at-age data for I. pygmaeus and L. chinensis revealed different patterns of growth during different seasonal periods. For L. chinensis growth was considerably slower during the winter period. For I. pygmaeus slower growth was also observed during the cooler period of the year. However, there was considerable scatter in the size-at-age data, probably as a result of greater variability in the near-shore environment. Differences were also detected in the relative statolith size in individuals of L. chinensis, with winter squids having larger statoliths for any given length than their summer counterparts. Alternatively, at comparable ages, winter squids had smaller statoliths than their summer counterparts, however, with increasing age, the longer-lived winter squids eventually obtained larger relative statoliths.

Determining individual size-at-age allowed for a more detailed understanding of age specific reproductive patterns. Male individuals of Loligo chinensis matured at a greater age in July (winter) than in January (summer) and no mature females were found in July. These age specific maturity patterns could not be discerned by an analysis of individual size. Development of the nidamental gland and hectocotylus was shown to follow a similar pattern to gonad development.

Mature individuals of L. chinensis were found throughout the year (except July, 1989 for females). However, there was considerable fluctuations in the gonadosomatic index for both sexes with largest relative gonad sizes achieved during October for both years studied. This pattern appeared to be real and not biased by variation in the size of individuals captured during different months of the year.

Idiosepius pygmaeus showed a different pattern in gonad growth with cooler season (spring) sepioids growing older and having larger gonads than warmer season (autumn) individuals, despite the fact that there was very little difference in individual size between the two seasons. Measurement of oocyte diameters within the ovaries of two I. pygmaeus females suggested that this species was a serial spawner. Maintenance experiments of captive I. pygmaeus females confirmed that this species lays its eggs in repeated batches, rather than all at once. Maximum egg output recorded was 922 eggs produced in 15 days. Moreover, histological examination of the oocyte size distribution of all five study species suggested that they were serial spawners, since a variety of oocyte sizes were present within the ovary.

More relevant tropical squid growth models could be developed as a result of obtaining individual size-at-age data. All the five study species displayed allometric growth, with Loligo chinensis and Loligo sp. 1 displaying significant differences in the length-weight relationship between males and females, while Idiosepius pygmaeus revealed sex-related differences in slope elevation of the L-W relationship.

Detailed growth models were developed for L. chinensis and S. lessoniana. Loligo chinensis were shown to exhibit exponential growth with a considerable proportion of the lifespan spent as juveniles. There was no significant difference between the growth curves of male and female L. chinensis in either growth in length or weight. The growth of S. lessoniana could not be modelled with a single equation. Therefore, an exponential curve was fitted to the size-at-age data (for both mantle length and weight) for the first 70 days, while growth from 70 to 153 days could be described by a linear equation. These growth models were compared to growth curves developed for other loliginids raised in captivity. Loligo chinensis was found to grow faster than other temperate loliginids, while the growth of S. lessoniana revealed a remarkable similarity to the form of growth of the tropical S. sepioidea. Length-at-age data for Loligo opalescens was found to not differ greatly from results of growth of this species in captivity, although field individuals appeared to grow somewhat faster than captive individuals.

Very clear growth rings were also observed within the statolith microstructure of the giant squid Architeuthis, which suggested that ageing research and growth modelling based on size-at-age data could be expanded to a variety of other species of oceanic squids. Statolith growth ring analysis is thus one means to obtain important growth parameters for squid species in which we have little biological information for.

Item ID: 24110
Item Type: Thesis (PhD)
Keywords: aging; Cleveland Bay; growth rates; Idiosepius; Loligor; Loliolus; reproduction; seasonal variation; Sepioteuthis; squid; Statoliths; Townsville Harbour
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Date Deposited: 13 Dec 2012 23:36
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 33%
06 BIOLOGICAL SCIENCES > 0608 Zoology > 060803 Animal Developmental and Reproductive Biology @ 34%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 33%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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