Size matters: large spiny lobsters reduce the catchability of small conspecifics

Ej Tuffley,T Langlois,J How,S De Lestang

doi:10.3354/meps13695

Abstract

Indices of lobster abundance and population demography are often derived from pot catch rate data and rely upon constant catchability. However, there is evidence in clawed lobsters, and some spiny lobsters, that catchability is affected by conspecifics present in pots, and that this effect is sex- and size-dependent. For the first time, this study investigated this effect in Panulirus cygnus, an economically important spiny lobster species endemic to Western Australia. Three studies: (1) aquaria trials, (2) pot seeding experiments, and (3) field surveys, were used to investigate how the presence of large male and female conspecifics influence catchability in smaller, immature P. cygnus. Large P. cygnus generally reduced the catchability of small conspecifics; large males by 26-33% and large females by 14-27%. The effect of large females was complex and varied seasonally, dependent on the sex of the small lobster. Conspecific-related catchability should be a vital consideration when interpreting the results of pot-based surveys, especially if population demography changes. Analysis of the mean catch rate of large P. cygnus over the past 29 yr indicates that fishery management changes have created significant variations in the abundance of large lobsters. This is likely to have affected the catchability of smaller lobsters, which will have implications for the use of time series catch rate data in the stock assessment and management of this fishery.

Highlights

As there is currently no recognised method to age lobsters, stock assessment and management of lobster fisheries rely heavily upon representative and consistent size- and sex-structured abundance data (Punt et al 2013)
Small lobsters sheltered with large males in only 24% of trials (Fig. 3; small lobsters (SL) = large mature males (LM)/SL, p = 0.016), indicating a 26% reduction from the null hypothesis of no preference (50%)
While not significant at the 95% confidence limit (p = 0.11), there was a tendency for small lobsters to avoid large females, with only 36% of the small lobsters choosing to shelter with the large female (Fig. 3; SL = large mature females (LF)/SL), a 14% reduction from the null hypothesis

Summary

Introduction

As there is currently no recognised method to age lobsters, stock assessment and management of lobster fisheries rely heavily upon representative and consistent size- and sex-structured abundance data (Punt et al 2013). Due to the depths across which many lobster species live and their cryptic nature, pots (or traps) are typically the most cost-effective (Miller 1990) and potentially the most accurate (Tuffley et al 2018) method to collect abundance and biological data on lobster populations. Spiny lobsters preferentially select dens housing conspecifics in the wild (Childress & Herrnkind 2001, Cobb 1981), and will choose to cohabit a den, rather than den alone, in experimental aquaria (Zimmer-Faust & Spanier 1987). This behaviour is so pronounced in the Caribbean spiny lobster that it is exploited in the Florida spiny lobster fishery, where live juvenile P. argus are used as attractants in place of bait (Hunt 2008). Attraction between conspecifics may mean that captured spiny lobsters increase the catchability of subsequent lobsters (Miller 1990)

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Conclusion