Managing a complex population structure: exploring the importance of information from fisheries-independent sources

Peer-reviewed. This is a pre-copy-editing, author-produced PDF of an article accepted for publication in ICES Journal of Marine Science following peer review. The definitive publisher-authenticated version is available online at: doi: 10.1093/icesjms/fsu102Natural resource managers aim to manage fish stocks at sustainable levels. Often, management of these stocks is based on the results of analytical stock assessments. Accurate catch data, which can be attributed to a specific population unit and reflects the population structure, are needed for these approaches. Often though, the quality of the catch data is compromised when dealing with a complex population structure where fish of different population units mix in a fishery. The herring population units west of the British Isles are prone to mixing. Here, the inability to perfectly allocate the fish caught to the population unit they originate from, due to classification problems, poses problems for management. These mixing proportions are often unknown; therefore, we use simulation modelling combined with management strategy evaluation to evaluate the role fisheries-independent surveys can play in an assessment to provide unbiased results, irrespective of population unit mixing and classification success. We show that failure to account for mixing is one of the major drivers of biased estimates of population abundance, affecting biomass reference points and MSY targets. When mixing of population units occurs, the role a survey can play to provide unbiased assessment results is limited. Either different assessment models should be employed or stock status should be considered from the survey data alone. In addition, correctly classifying the origin of fish is especially important for those population units that are markedly smaller in size than other units in the population complex. Without high classification success rates, smaller population units are extremely vulnerable to overexploitation.Funder: Marine Institut

Production of soma and gonad in maturing male Illex argentinus (Mollusca: Cephalopoda)

Samples of male Illex argentinus were taken from the catch of a Japanese squid jigging vessel on the Patagonian Shelf during March 1986 and an analysis was carried out on the morphometrics of the somatic and reproductive organ systems in relation to maturation. The data show that growth and maturation occurred simultaneously during most of the time that Illex argentinus males were on the feeding grounds over the southern Patagonian Shelf. In a squid of a ‘standard’ mantle length the whole body mass increased relative to mantle length during maturation and this could be attributed to the increase in mass of the reproductive and accessory reproductive organs. During maturation the mantle and digestive gland mass showed no significant change relative to mantle length. The mass of the head increased and the mass of the viscera decreased relative to mantle length. In male Illex argentinus, as in the female, the energy and nutrient resources for maturation are derived from the squid's food and during the course of maturation there is an increasing shift of emphasis from somatic growth to production of gonad and accessory reproductive organs. The proportional investment of body mass in reproductive and accessory reproductive organs predicted for a fully mature male Illex argentinus was less than half that of the female

Dynamics of growth and maturation in the cephalopod Illex argentinus de Castellanos, 1960 (Teuthoidea: Ommastrephidae)

Post-recruit Illex argentinus were collected from the fishery on the Patagonian Shelf between 1986 and 1988. Age was determined by analysis of daily growth increments in ground sections of the statolith, female fecundity was determined, specimens were dissected, weighed and assigned a maturity stage. The relation between mantle length and age is best approximated by a linear model. Both sexes live approximately one year. Females grow faster and attain a larger size than males, but males mature younger. Hatching occurs during the austral winter, peaking in June-July. There is a consistent trend of increasing growth rate with later date of hatching. Statolith growth is negatively allometric with body growth and there is greater divergence between statolith and body growth in females. In post-recruit Illex growth in mass of somatic tissues, apart from the digestive gland, approximates isometry with growth of the whole body. Growth of the digestive gland and the reproductive organs is positively allometric with growth in whole body mass. There is poor correlation between the mass of female reproductive organs and whole body mass, which is due to differences in size at maturity. Mating rarely occurs on the feeding grounds. Fecundity of fully mature females in the sample falls in the range 113835-246098 eggs per individual. This possibly underestimates average fecundity because mature squid on the feeding grounds may be precocious and smaller than average at full maturity. A model combining absolute and relative growth, predicts average male and female growth and growth of the major somatic and reproductive organs. This poorly predicts average female maturity indices with age because of variability in mass of the female reproductive organs. A model that treats maturity stages separately gives close agreement with measured mass of the reproductive organs and maturity indices of a mature female at age one year. Females invest approximately 20% of total body mass in gonad and accessory reproductive organs at full maturity

Age determination in squid using statolith growth increments

Growth increments have been reported to occur in the squid beak, radula, gladius and statolith. Of these, the statolith, which is part of the organ responsible for detection of linear and angular acceleration, has proved most promising for age determination. Growth increments in the statolith are formed from aragonite crystals in an organic matrix. They are best viewed after sectioning the statolith or after decalcification in weak acid. The statolith grows in concert with the rest of the squid. Experiments with squid in which chemical markers have been incorporated at a known time in the statolith, and experiments with cultured squid of known age, appear to confirm the hypothesis that growth increments in the statolith are laid down daily. Increments are produced in the laboratory in the absence of tidal, feeding or temperature cycles, which suggests that there is a firmly entrained endogenous circadian rhythm associated with their formation. However, the possibility that increment formation can be disrupted by environmental factors, or that rings in the statolith are produced coincidentally at the rate of approximately one per day, should not be fully discounted without further experimental corroboration. Data on squid age, derived from growth increments in the statolith, clearly have value in fisheries investigations, but they should be treated with caution until they have been validated

Production of soma and gonad in maturing female Illex argentinus (Mollusca: Cephalopoda)

Samples of female lllex argentinus were taken from the catch of a Japanese squid jigging vessel on the Patagonian Shelf during March 1986. Morphometrics of the somatic and reproductive organ systems and the histological structure of the mantle in relation to maturation were examined. The data suggest that growth and maturation occur simultaneously during most of the time that lllex argentinus females are on the feeding grounds. In a squid of a ‘standard’ mantle length the whole body mass increases relative to mantle length during maturation and growth of the reproductive organs. This is accompanied by a small but significant decrease in the relative mass of the mantle, head and viscera whilst the mass of the digestive gland remains constant. Although mantle mass of a ‘standard’ female squid decreases relative to mantle length with maturity this is not associated with degeneration of the mantle muscles. Energy and nutrient resources for maturation are apparently derived from the squid's food, not from reserves, and during the course of maturation there is an increasing shift of emphasis from somatic growth to production of gonad and accessory reproductive organs

Demography and distribution of the Patagonian squid (Loligo gahi d'Orbigny) during the austral winter

A bottom-trawl survey of the Falklands Interim Conservation Zone was carried out during July and August 1988. Samples of Loligo gahi were collected from depths of 50–400 m and analysed for size composition, maturity, and stomach fullness. L. gahi increased in size and maturity with depths >100 m. Immature and fully mature squid were found at depths ≤100 m. The data support the hypothesis that this species spawns in shallow water and migrates down the continental shelf and continental slope after hatching. It then returns to shallow water to spawn. Highest concentrations of squid appeared to occur at depths and in areas where the commercial fishery operates. Samples were only taken during daylight hours, and stomach fullness was at its highest in the mid-afternoon. Several of the life history features resemble closely those of other loliginid species

Recruitment, demography and growth of the Patagonian squid Loligo gahi (D'Orbigny, 1835) in Falkland Island waters

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