Changes in growth of juvenile southern bluefin tuna (Thunnus maccoyii): an analysis of length-frequency data from the Australian fishery

Abstract

Modal analysis is applied to historical length–frequency records of the Australian southern bluefin tunafishery, in order to quantify the variation in mean length from year to year. In the South Australian fishery in the first half of March, the mean length has ranged between 54 cm and 64 cm for 1-year-old fish, 73 cm and 85 cm for 2-year-old fish, and 85 cm and 100 cm for 3-year-old fish. The mean lengths of 2-, 3- and 4-year-old fish, and the increment from age 1 to age 3, have increased substantially over the history of the fishery. This increase in growth is probably a response to a decline in the population due to heavy fishing. In many years in the Western Australian fishery, two or more groups of 1-year-old fish were found: the mean lengths of these groups typically differed by 10 cm. Growth rates also varied markedly according to the season of the year.

Modelling growth in tuna RFMO stock assessments: Current approaches and challenges

“…Laslett et al, 2002), or the estimation may be done in a single combined step, in which the means from the modal decomposition are constrained to follow a given growth curve (e.g. Hampton, 1991;Leigh and Hearn, 2000).…”

Fitting growth models to length frequency data

“…Our method is a two-stage analysis in which a modal decomposition is performed in the first stage without making any assumptions about the pattern of growth. Single-stage methods, such as MULTIFAN and that of Leigh and Hearn (2000), constrain the modal estimates to lie on (or near) a parametric growth curve. In our experience, length frequency growth patterns do not conform tightly to parametric growth curves.…”

“…Our method has some features in common with other methods presented in the quantitative fisheries literature (Fournier et al, 1990;Leigh and Hearn, 2000), but departs from them in significant ways. In particular, we develop a two-stage analysis.…”

“…For a given half-month, suppose there are m i fish of length l i for i ¼ 1; . ; n. Here and elsewhere, m i is the recorded catch count divided by the Leigh and Hearn (2000) weighting factor. The lengths are assumed to be generated from a K component mixture in which the probability density of component k is g k (l i ), where…”

“…Standard error estimates from treating the recorded data as raw data would, in general, be too small. To correct for this, we adopted the weighting factors derived in Appendix 1 of Leigh and Hearn (2000), which are based on the variance of the mean appropriate for the two-phase sampling procedure. Let j denote the weighting factor for a given halfmonth.…”