Abstract
The demographics of the southern bluefin tuna (SBT) Thunnus maccoyii spawning stock were examined through a large-scale monitoring program of the Indonesian longline catch on the spawning ground between 1995 and 2012. The size and age structure of the spawning population has undergone significant changes since monitoring began. There has been a reduction in the relative abundance of larger/older SBT in the catch since the early 2000s, and a corresponding decrease in mean length and age, but there was no evidence of a significant truncation of the age distribution. Pulses of young SBT appear in the catches in the early- and mid-2000s and may be the first evidence of increased recruitment into the spawning stock since 1995. Fish in these two recruitment pulses were spawned around 1991 and 1997. Size-related variations in sex ratio were also observed with female bias for fish less than 170 cm FL and male bias for fish greater than 170 cm FL. This trend of increasing proportion of males with size above 170 cm FL is likely to be related to sexual dimorphism in growth rates as male length-at-age is greater than that for females after age 10 years. Mean length-at-age of fish aged 8–10 years was greater for both males and females on the spawning ground than off the spawning ground, suggesting that size may be the dominant factor determining timing of maturation in SBT. In addition to these direct results, the data and samples from this program have been central to the assessment and management of this internationally harvested stock.
Highlights
High fishing pressure can affect the size and age structure of a fish population, often resulting in a reduction in the relative abundance of larger and, indirectly, older individuals [1]
Results from the generalized linear models (GLMs) suggest that spawning season, month and BE index are all highly significant explanatory variables of the proportion of small Southern bluefin tuna (SBT), as are the pair-wise interaction terms between spawning season and month and spawning season and BE
SBT on the spawning ground increased between seasons 2000 and
Summary
High fishing pressure can affect the size and age structure of a fish population, often resulting in a reduction in the relative abundance of larger and, indirectly, older individuals (age-class truncation) [1]. This suggests that, in lieu of survey data from which a more complete estimate of the size distribution on the spawning ground could be derived, SBT caught in shallow longline sets may better represent the spawning population This long-term monitoring of the Indonesian fishery and collection of large quantities of high quality length and direct age data has allowed us to estimate the size and age distribution of the Indonesian catch over the past two decades, examine changes in size/age based parameters over time, and estimate sex ratio and sex specific growth rates of the spawning population. It has allowed us to examine whether there have been changes in fishing practices, such as depth of fishing of the Indonesian fleet, which
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