Could warming oceans and increased lobster biomass rates be affecting growth rates in Australia's largest lobster fishery?

Recent increases in water temperature and lobster density have been identified as potential factors that may be impacting Australia's most valuable single-species commercial fishery. This situation is not limited to Western Australia, with many other lobster fisheries worldwide experiencing similar changes. To examine this, growth rates of Panulirus cygnus George, 1962 were modeled using three different relationships to describe variation in 25 yrs of tag- recapture data. An inverse-logistic function provided the best fit to the data, with the more commonly used Faben's technique being limited by its assumption of an upper asymptotic length, a characteristic not exhibited by the data. The modelling technique facilitated the inclusion of growth covariates, namely sex, water temperature, population density, movement behavior, and geographic position. Migration was found not to significantly impact lobster growth rate, while its sex, water temperature, population density, and geographic position (coastal or islands) all significantly impacted growth rates. Female lobsters grew slower than males, warmer water temperatures correlated with increased growth rates in subadults, and a subsequent earlier onset of slow adult growth rates. Increased population densities and occupying the offshore waters of the Abrolhos Islands were both significantly negatively correlated with growth rates. With the coastal waters in south Western Australia progressively warming and legal lobster densities being more than five times greater now than a decade ago, growth rates of lobsters have declined since the 1990s. The impact of variable growth dynamics should be incorporated into stock assessment modelling for lobster fisheries experiencing changing environment conditions.