Vol. 666: 99–113, 2021 MARINE ECOLOGY PROGRESS SERIES Published May 20 https://doi.org/10.3354/meps13695 Mar Ecol Prog Ser OPEN ACCESS Size matters: large spiny lobsters reduce the catchability of small conspecifics Emma-Jade Tuffley1,2,3,*, Simon de Lestang2, Jason How2, Tim Langlois1,3 1School of Biological Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia 2Aquatic Science and Assessment, Department of Primary Industries and Regional Development, 39 Northside Drive, Hillarys, WA 6025, Australia 3The UWA Oceans Institute, Indian Ocean Marine Research Centre, Cnr. of Fairway and Service Road 4, Crawley, WA 6009, Australia 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 cyg nus, 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 investi- gate 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 demo graphy 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 abun- dance 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. KEY WORDS: Panulirus cygnus · Spiny lobster · Catchability · Intraspecific interactions · Behaviour 1. INTRODUCTION catchability remains constant be tween samples and individual lobsters, or that its variation can be accu- As there is currently no recognised method to age rately accounted for (Morgan 1974). lobsters, stock assessment and management of lobster For a variety of lobster species, there is mounting fisheries rely heavily upon representative and consis- evidence of size- and sex-dependent catchability re - tent size- and sex-structured abundance data (Punt et lated to conspecific interactions in and around pots. al. 2013). Due to the depths across which many lobster In clawed lobsters, underrepresentation of small ani- species live and their cryptic nature, pots (or traps) are mals in catch data is common and usually attributed typically the most cost-effective (Miller 1990) and po- to antagonistic encounters with larger animals in and tentially the most accurate (Tuffley et al. 2018) around pots (Miller 1989, 1995, Addison 1995, Addi- method to collect abundance and biological data on son & Bannister 1998). While antagonistic behaviours lobster populations. Using catch rates from pots to have been observed in numerous spiny lobster spe- make inferences about the abundance, composition, cies, including the Japanese spiny lobster Panulirus or state of a stock is reliant on the assumption that japo nicus (Hamasaki et al. 2018), the western rock © The authors 2021. Open Access under Creative Commons by *Corresponding author: Attribution Licence. Use, distribution and reproduction are un - emma-jade.tuffley @research. uwa. edu.au restricted. Authors and original publication must be credited. Publisher: Inter-Research · www.int-res.com 100 Mar Ecol Prog Ser 666: 99–113, 2021 lobster P. cygnus (Cobb 1981), the southern rock eries typically target specific size−sex classes, often lobster Jasus edwardsii (MacDiarmid 1989), and the large males in lobster fisheries, fishing pressure can Caribbean spiny lobster P. argus (Shabani et al. lead to changes in population demography (Iacchei 2009), the possible impacts of these interactions on et al. 2005, Barrett et al. 2009, Bellchambers et al. catch rates have been less well studied than in 2017). For example, the removal of large J. edwardsii clawed lobsters. during the fishing season in South Australia has been Additionally, unlike clawed lobsters, spiny lobsters shown to increase trap selectivity for small lobsters are gregarious, and therefore the effects of conspe- to wards the end of the season (Frusher & Hoenig cific interactions on the catchability of spiny lobsters 2001). This effect is so great that it masked declining are potentially more complicated. Spiny lobsters pre - stock recruitment over a 35 yr period in that fishery ferentially select dens housing conspecifics in the (Frusher et al. 2003). Similarly, changes in fishery wild (Childress & Herrnkind 2001, Cobb 1981), and management, such as the adoption of more conserva- will choose to cohabit a den, rather than den alone, in tive targets or the implementation of fishery closures experimental aquaria (Zimmer-Faust & Spanier or no-take zones, can lead to increased abundances 1987). This behaviour is so pronounced in the Carib- of the most targeted sex−size classes (Cox & Hunt bean spiny lobster that it is exploited in the Florida 2005, Babcock et al. 2007, Montgomery & Liggins spiny lobster fishery, where live juvenile P. argus are 2013). If large lobsters deter small lobsters, increased used as attractants in place of bait (Hunt 2008). At - abundance of large individuals may reduce the traction between conspecifics may mean that cap- catchability of smaller lobsters, which may lead to tured spiny lobsters increase the catchability of sub- underestimating the abundance of small lobsters and sequent lobsters (Miller 1990). potentially misleading interpretations around the The only spiny lobster species to have received performance of closed areas. sub stantial investigation of conspecific-related catch - Catch per unit effort (CPUE) from pots is an impor- ability is the southern rock lobster J. edwardsii. In this tant data source for the management of the P. cygnus species, catch rates of large and small individuals fishery. P. cygnus is endemic to Western Australia, have been negatively correlated (Frusher & Hoenig where it supports Australia’s most valuable single- 2001, Ziegler et al. 2002), and large male and female species fishery, the West Coast Rock Lobster Man- lobsters inhibited small conspecifics from entering aged Fishery (WCRLMF), worth approximately AUD pots in pot seeding experiments (Ihde et al. 2006). $400 million annually (Gaughan & Santoro 2018). These effects were seasonal and varied by sex, with Management of this fishery is governed by stock as - the strongest inhibitory effect occurring between sess ment models which use, in addition to other data large and small females during the mating season. sources, pot-based CPUE data from annual in de pen - Similarly, research suggests den cohabitation patterns dent breeding stock surveys run by the state’s fish- in the same species are also seasonal and dependent eries department (the Department of Primary Indus- on sex. Large reproductive males were aggressive to- tries and Regional Development; DPIRD). In the wards other males and rarely cohabited during the 2008− 2009 fishing season, low levels of puerulus set- mating season, and likewise, cohabitation in large re- tlement prompted dramatic management changes, productive females was also lowest during this period which led to a significant reduction in the fishery’s (MacDiarmid 1994). These antisocial behaviours oc- annual catch (de Lestang et al. 2016, Caputi et al. curred exclusively within sexes, as rates of cohabita- 2018). This conservative level of fishing was main- tion between sexes were greatest during this period tained as the fishery adopted maximum economic (MacDiarmid 1994). This increased cohabitation could yield as its target in the harvest strategy. As a result, lead to pot attraction, and correspondingly, there was the legal-sized biomass of this fishery approximately some evidence of a seasonal attractant effect between doubled over a 10 yr period and is now the largest it large male and small female J. edwardsii in pot seed- has been for several decades (de Lestang et al. 2016, ing experiments (Ihde et al. 2006). 2018, Caputi et al. 2018). In addition to an overall in - Any deterrent or attractant effect of lobsters in and crease in biomass, these management changes have around pots will affect the catchability of subsequent likely changed the demographics of the fishery, with lobsters. As these effects are often size- and sex- a greater number of large lobsters being present. As specific, spatial or temporal variation in population there has been little research on the intraspecific demography will cause variation in catchability, and interactions of P. cygnus in and around pots, it is subsequent variations in size−sex structured abun- unknown what effect the increased relative abun- dance data calculated from catch rates. Because fish- dance of large animals will have on pot-based size- Tuffley et al.: Conspecifics affect lobster catchability 101 and sex-structured catch rate data used in stock 2. MATERIALS AND METHODS assessment modelling. This study