The Pribilof Islands blue king crab stock was closed to directed harvest in 1999, declared overfished in 2002, and is the only overfished stock in the North Pacific. This fishery closure combined with habitat protection and bycatch reduction measures have failed to rebuild the stock. The 2015 Stock Assessment and Fishery Evaluation index of stock biomass was 8% of maximum sustainable yield (BMSY) with no indication of recruitment. We propose to investigate if larval supply, habitat availability, or predation on juveniles are limiting recruitment to inform potential future rebuilding efforts. Our field study in the Pribilof Islands will examine three potential causes of recruitment limitation, including larval supply, habitat limitation, and predation. Larval collectors and dive surveys will quantify abundance and distribution of newly settled and juvenile king crabs in nearshore habitats to test the role of larval supply. Dive surveys and drop cameras will characterize habitat in nearshore areas, which will be compared to historical data to identify any changes over time in habitat availability. Dive surveys, fish stomach contents and in situ tethering experiments will quantify predation pressure. This multifaceted approach will address the mechanisms controlling the abundance of early benthic phase Pribilof Islands blue king crabs and evaluate if a lack of juvenile recruitment is occurring and limiting recovery in the fishery. While we plan to focus on blue king crab, we will collect information on any red king crab encountered in the study as well. The results of this study will thus be applicable for management of Bering Sea blue and red king crab stocks by providing baseline data regarding crab recruitment, habitat and predation. Our proposed empirical studies specifically address NPRB research priorities on data poor and depressed stocks and will be a significant contribution to understanding the processes controlling crab recruitment dynamics.
- Our overall objective is to investigate if juvenile recruitment limitation and a bottleneck in larval and juvenile stages are occurring and limiting rebuilding efforts of Pribilof Islands blue king crab.
- Quantify larval supply and early juvenile abundance.
- Resample habitat from historical surveys and identify availability of habitat in shallow areas.
- Identify potential juvenile king crab predators and investigate predation potential.
- Identify distribution and overlap of red and blue king crab juveniles.
- Communicate meaningfully and engage with local residents in research and communicate our results to fishery managers to inform fishery management and rebuilding efforts.
|Background and Concept|
The lack of recovery of Pribilof Islands blue king crab is likely complex and not due to any one single factor. Possible causes are described below.Lack of larval supply
Because crabs have a complex life cycle with dispersive larval stages and a sedentary adult, successful larval recruitment is required. Blue king crab have a limited distribution in the Bering Sea (Figure 3b), which requires larval connectivity among disjointed populations or retention within a local region. In the Pribilof Islands adult blue king crab females likely release larvae in nearshore habitat (Armstrong et al. 1987). The general anti-cyclonic, circular water flow around the Pribilof Islands (Kowalik & Stabeno 1999, Stabeno et al. 2008) could promote retention of locally hatched larvae, but whether these currents promote retention across years and seasons is unknown. Larval transport is currently being studied via a NOAA Fisheries and the Ecosystem (FATE) grant (Foy et al. 2013), where ROMS circulation models are combined with individual based models to track particle movement in different years. This ongoing study will examine the potential for retention under past and current oceanographic conditions. Research is needed to examine larval supply in the field.
Northeast Pacific marine communities and populations undergo systematic re-organization and change on inter-decadal time scales (Francis & Hare 1994, Anderson & Piatt 1999) that may be climatically driven (Miller et al. 1994, Francis et al. 1998, Hare & Mantua 2000, Bond et al. 2003). The collapse of several Alaskan crab fisheries, particularly red king crab, coincided with decadal changes in climate (Zheng & Kruse 2000), and hypotheses linking climate and fishery declines include changes in larval advection and survival (Tyler & Kruse 1996, 1997) and increased disease and/or predation. Coherence in declines across larger geographic areas and across species support a link between climate and crab stocks (Orensanz et al. 1998). Shifts in climate may be causative factors for changes in community structure and trophic linkages. Shifts in climate and oceanography may be advantageous for predators that remove small juvenile crabs, preventing population recovery (Zheng & Kruse 2006). Correlational studies of biological populations and climate are the subject of speculation, because specific mechanisms of mixing and circulation on scales relevant to biological populations are lacking (Sinclair & Frank 1995) and other factors such as recruitment pulses or fishing-down stocks may have coincided (Orensanz et al. 1998). Recruitment of mature male blue king crab stock to the fishery is highly variable (Figure 1). King crab fishery recruitment is hypothesized to be stronger during cold seawater temperature periods and weaker during warm periods across Alaskan waters (Zheng & Kruse 2000). Blue king crab recruitment trends are similar between the St. Matthew Island and Pribilof Islands areas and likely exist under the same regional climate and trophic influences (Figure 3b, Zheng & Kruse 2006). Changing temperatures could alter predator-prey dynamics or competitive interactions (Long et al. 2015, Lyons et al. 2015). Little research has been done to examine potential mechanisms linking climate and regime shifts with blue king crab.
Juvenile nursery habitat
Cobble and shell hash substrates are a preferred settlement habitat for blue king crab glaucothoe (Tapella et al. 2009) and were available near the Pribilofs Islands in 1983-84 during a field study of nearshore habitat associations that documented high abundance of blue king crab juveniles in shell hash around St. Paul and St. George Islands (Figure 3, Armstrong et al. 1987). These data were the focus of a recent NPRB funded data rescue project (NPRB # 1321) and are now available for comparison with current day to investigate if changes in habitat may be limiting juvenile recruitment. For young juveniles, complex habitat is critical to avoid predation (Daly & Long 2014a) and is likely the driving factor in the distribution of juveniles (Armstrong et al. 1987). An acute disappearance of a strong juvenile cohort detected in 2005 (Daly et al. 2015) suggests that juvenile survival may be a bottleneck.
Predation and red king crab competition
As described above in the regime shift section, an increase in predators could reduce the survival of juvenile blue king crab. However, there are currently almost no data on which species are important blue king crab predators or how habitat affects predation risk in situ. Interactions between juvenile red king and blue king crab may have ramification for juvenile blue king crab survival, including competition and potentially predation by red king crab juveniles. Red king crab generally prefer vertically complex, epiphytic habitat. Conversely, blue king crab maintain a more cryptic lifestyle and burrow into shell hash habitats where they are cryptic (Figure 4, Daly & Long 2014a,b). Inter and intra-species predation may be mediated by complex (shell hash) habitat; however, cohort competitive advantage and antagonistic behavior favor red king crab (Long et al. 2015, Daly & Long 2014a). Negative interactions favoring red king crab may therefore be a limiting factor for blue king crab stock recovery (Halley et al. 2004, Long et al. 2015). This study will focus on assessing these laboratory results and assumptions in the field, by ascertaining the degree of habitat overlap between the two king crab species and confirmation of predator abundances and likely interactions.
In this study we will close some of the gaps in our understanding of blue king crab larval supply, post-settlement survival, availability of habitat, and predation potential. We will quantify larval supply using larval collectors to determine if larval supply to critical habitat types is a bottleneck in the life history. We will assess shallow habitat distribution and juvenile densities through transect and quadrat-based SCUBA dive surveys at 5-20 m depths where substrate composition will be characterized by percent cover, grain size, estimated patch size, biogenic composition (shell hash characteristics), and rugosity (roughness or fractal dimension) (Coyer and Witman 1990, Beck 2000, Long et al. 2015). Additionally, habitat distribution in deeper areas will be compared with historical surveys (Armstrong et al. 1987) through spot-checking habitat using inexpensive drop cameras (Rooper 2008). We will quantify predator and competitor species abundance (including juvenile red king crab) in blue king crab habitat during dive surveys. Blue king crab predators will be identified using video camera monitoring of tethered juvenile king crab (Pirtle et al. 2012, Daly et al. 2013) along with sampling of fish stomach contents. Combined the surveys, tethering and stomach content analysis will identify the predation potential to limit juvenile recruitment of blue king crab.