Benefits derived from opportunistic survival-enhancing interventions for the Hawaiian monk seal: the silver BB paradigm

Abstract

The cumulative benefits derived from historic small-scale, opportunistic interventions for the Hawaiian monk seal Monachus schauinslandi were assessed using multiple methods. The analysis focused on interventions undertaken to enhance survival of individual seals by reducing or eliminating immediate mortality risks. These interventions included dehookings, disentanglements, removing seals from high predation zones, medical interventions, and related activities. A total of 885 interventions occurred range-wide from 1980 to 2012. These included 645 interventions classified as mitigating medium- to high-risk threats, involving 532 different seals. In the Northwest Hawaiian Islands, where most of these interventions took place, we found a significant relationship between the number of interventions conducted annually and duration of field effort. The survival and reproduction of the intervention seals were tracked through multiple generations, using (1) known survival and reproduction of intervention seals, and (2) expected survival and reproduction as determined using demographic rates estimated for the population at large. This analysis indicated that 17-24% of the 2012 population comprised either intervention seals or descendants of intervention seals. If seals included in a multiyear (1984-1992) rehabilitation and captive care effort are also included, this proportion increases to 32%. These findings demonstrate the important link between the sustained population assessment field effort, the number of interventions that are enabled in association with those efforts, and the current status of the monk seal population. In contrast to a metaphorical ‘silver bullet’ whereby a result is achieved through a single (or a few) highly impactive tools, we liken our success in applying multiple interventions to a fusillade of many silver BBs1.