Upward adjustments to local catch limits for the krill fishery in Subarea 48.1

Here we compile three vignettes that, together, provide the basis for making upward adjustments to local catch limits for the krill fishery in Subarea 48.1.  We use the term local catch limit to refer to a catch limit that applies to a group of SSMUs (gSSMU), and the work presented here is based on the gSSMUs defined in another compilation of vignettes (AERD 2016a, pp. 3-13).  We propose upward adjustments to local catch limits as one component of a larger strategy for feedback management (FBM) of the krill fishery in Subarea 48.1 (see Watters et al. 2016).  Our proposal to make upward adjustments is founded on 1) a “stoplight” classification of predator performance to identify years when predator performance is good and suggest that upward adjustments of local catch limits may not negatively impact predator populations, 2) repeat acoustic surveys to identify years when local krill biomass increases during the fishing season, and 3) a decision rule that can be used to increase local catch limits when the stoplight is “green” and there has been a concomitant increase in local krill biomass. We demonstrate that a simple index based on normalized CEMP parameters can be used to categorize predator performance as good (“green-light”) or poor (“red-light”). The approach is relatively insensitive to missing values, agrees generally with expert opinion, and suggests that indices of foraging trip duration, fledge weight, and reproductive success are highly influential in determining a green- or red-light classification of summer predator performance. We then show that local estimates of krill biomass around the Antarctic Peninsula are spatially and temporally correlated. Such patterns suggest that repeat surveys of standard transects within gSSMUs can be used to characterize larger-scale patterns of krill abundance and detect within-season increases in krill biomass. Thus, a ratio of late to early summer krill biomass estimates can be indicative of “surplus” krill potentially available to the fishery. To capitalize on such surpluses, we propose a step-change decision rule to increase local catch limits.  Provided the stoplight is green, the ratio of late-summer to early-summer estimates of krill biomass from acoustics surveys provides a simple catch-limit multiplier that can increase a local catch limit if the biomass ratio is greater than one. We advocate real-life testing of the ideas proposed here to prove the concept; in particular it would be useful for fishing vessels to collect acoustic data on repeat transects twice per summer.