Modelling size-at-age in wild immature female octopus: a bioenergetics approach

Abstract

The population dynamics of cephalopods are poorly understood because intra-specific size-at-age is characteristically variable. Much of the variation observed is attributed to temperature and food, but other generally overlooked factors such as hatchling size and inherent growth capacities also affect size-at-age. In the present paper, we investigated the relative influence of the principal abiotic (environmental temperature) and biotic (food consumption, hatchling size, inherent growth capacity) factors affecting size-at-age in immature octopus. Using a bioenergetics model and size-at-age data of wild-caught immature Octopus pallidus, we simulated the juvenile growth trajectories of individuals hatched in different seasons (summer, autumn and winter) based on food availability, metabolism, environmental temperature and individual variability, under an assumption of 2-phase growth. Simulations predict that the effect of hatchling size on size-at-age was secondary to that of inherent growth capacity. Projections suggest that wild immature populations comprise a mixture of individuals displaying exponential growth and 2-phase growth and that the proportion of each depends primarily on the individuals' inherent growth capacities and food availability. High food intake was projected to decrease the number of individuals displaying 2-phase growth by delaying the transition between the 2 growth phases, resulting in larger individuals. Overall, individuals hatched in summer grew to larger sizes and matured earlier than individuals hatched in autumn or winter, independent of food availability. The size-at-age distribution of the summer and autumn cohorts tended to become bimodal under certain food intake levels, which highlights the importance of coupling size data with accurate age estimates in future octopus population studies. KEY WORDS: Early life history · Energy balance · Individual-based model · Pale octopusResale or republication not permitted without written consent of the publisher

A multilevel approach to examining cephalopod growth usingOctopus pallidusas a model

“…Furthermore, if a day-old hatchling, which weighs about 0.2g, grew exponentially at 1.6% BM day -1 , it would only weigh 2.3g after 156days or 64g after 12months, supporting the suggestion of substantially faster growth during the early phase of the life cycle. Interestingly, modelled juvenile growth trajectories of O. pallidus, based on size-at-age data of wild immature females, suggest that O. pallidus populations comprise individuals with single-phase exponential growth and others with two-phase growth, with the proportions of each growth pattern influenced predominantly by food availability and the inherent growth capacity of the individual (André et al, 2009b). This may explain the differences in growth rates between this study and that of Leporati and colleagues (Leporati et al, 2007).…”

“…Other studies have also noted significant variation in initial size at hatching, growth and size-at-age between individual octopuses reared under identical laboratory conditions (Forsythe, 1984;Forsythe and Hanlon, 1985;Leporati et al, 2007 growth and size-at-age at an individual level can be highly influenced by environmental variables such as temperature (Forsythe and Hanlon, 1988;Segawa and Nomoto, 2002) and food availability (O'Dor et al, 1980), because of the nature of exponential growth, inherent growth plasticity and hatchling size heterogeneity can also lead to considerable differences in size over time. Modelled growth projections of O. pallidus suggest that inherent growth capacities have a strong influence on size-at-age, as does initial hatchling size (André et al, 2009b). Initial hatchling size was also found to influence growth rate in laboratory-raised juvenile O. pallidus, although the effects were secondary to temperature (Leporati et al, 2007).…”

“…Leporati et al, 2008a;Leporati et al, 2008b). Because of the nature of exponential or non-asymptotic growth, small inherent individual variations in growth rates or initial size at hatching may be magnified over time to produce large differences in size-at-age within a population (André et al, 2009b). Laboratory culture studies have consistently suggested that cephalopod growth occurs in two phases over the life cycle (e.g.…”

“…Laboratory culture studies have consistently suggested that cephalopod growth occurs in two phases over the life cycle (e.g. DeRusha et al, 1987;Forsythe and Hanlon, 1988;Segawa and Nomoto, 2002), which typically consists of an early phase of rapid exponential growth followed by a slower phase, best described by a power equation (Semmens et al, 2004); however, there is considerable debate about whether twophase growth occurs in the wild (André et al, 2009b). These unique growth characteristics have important implications for a species' population dynamics and productivity and how they respond to environmental change and variability.…”

Octopus life history relative to age, in a multi-geared developmental fishery

“…To help solve this issue and get a better understanding of the dynamics of the population, pre-recruitment sampling is required with emphasis on paralarval distribution and abundance patterns in context to a range of biotic and abiotic factors (e.g. salinity, habitat, oceanographic conditions) (André et al, 2009).…”