Indirect effects of recreational fishing on spiny lobster (Panulirus argus) behavior, mortality and population dynamics.

Abstract

The most obvious impact of a fishery is the large amount of biomass removed. Fishing also impacts and disturbs animals that are not caught, but interact with fishing gears. These indirect effects of fishing often go unnoticed but may be an important component of the overall ecosystem modification for which fisheries are responsible.

In this study, we quantified indirect effects within the Caribbean spiny lobsters (Panulirus argus) fishery in the Florida Keys, USA, including mechanisms that resulted in behavioral modification and lobster mortality, and potential population and economic consequences of these indirect effects. The spiny lobster fishery in the Florida Keys has intense recreational sport-diver effort during a 2d mini-season in which 80-90% of legal lobsters are removed from some areas, and an over capitalized commercial trap fishery where >900,000 lobster traps are deployed annually.

In field surveys we observed a 27% increase in the density of injured lobsters on patch reef habitats from before to after the 2 d mini-season, but not in other habitat types. In laboratory Y-maze shelter choice experiments, we observed that un-injured lobsters were less likely to shelter with injured lobsters than with un-injured lobsters. Using disturbance experiments conducted in the field we observed that disturbing and injuring lobsters on a coral patch-head caused individuals to emigrate from that patch-head before the mini-season, but had no effect on lobster density on patch heads after the mini-season. When lobsters were tethered in the field, mortality was higher when injured than un-injured. Similarly, lobsters that were exposed to triggerfish predators (Balistes capriscus) in a large seawater arena had a higher probability of mortality after being disturbed and injured.

The behavioral response of lobsters exposed to human disturbance and the predation threat presented by triggerfish were compared in the same seawater arena mentioned above. Both human disturbance and the presence of triggerfish decreased lobster fidelity to individual shelters. Overall shelter use and lobster gregariousness increased when triggerfish were present, but not as a function of human disturbance. Lobsters may not always respond to human disturbance and natural predators with similar behaviors.

We used an individual-based population model to quantify potential population and economic impacts of injuries lobsters receive from sport-divers in the Florida Keys. Model results suggest that the severity of population and economic impacts are dependent on the frequency of lobster injuries throughout the Florida Keys fishery. For example, areas associated with high densities of injured lobsters may have a ≥50% reduction in the abundance of adult lobsters and associated recreational and commercial harvests. In areas where the density of injured lobsters was ~20 times lower, the impact of injuries on the adult lobster population and commercial landings was only 5 and 8%, respectively. Despite the seemingly smaller impact in areas with lower densities of injured lobsters, injuries may still be responsible for a 900 t reduction in the adult stock biomass and a 160 t, or $1.6 million reduction in annual commercial landings when scaled to the entire spiny lobster fishery in the Florida Keys.

Sub-lethal interactions with fishing gears can lead to altered behavioral responses by injured animals, altered interactions with other members of the community, and ultimately, increased mortality. These potentially cumulative mortalities have potential population and economic consequences. The occurrence of unobserved mortalities resulting from fishing activities is probably not restricted to the spiny lobster fishery in the Florida Keys. More likely, similar undesirable and costly consequences of unobserved mortalities occur in fisheries throughout the world. To avoid these costs, it would be prudent to account for unobserved mortalities in all fisheries.