Objectives

Study Objectives

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.

Project Objectives:

  1. 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.
  2. Quantify larval supply and early juvenile abundance.
  3. Resample habitat from historical surveys and identify availability of habitat in shallow areas.
  4. Identify potential juvenile king crab predators and investigate predation potential.
  5. Identify distribution and overlap of red and blue king crab juveniles.
  6. 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

Figure 1. Historical Pribilof Islands area blue king crab estimated mature male biomass (black, line) and fishery catch (red, open circles) from 1973 to 2015 (NPFMC 2015).

Figure 1. Historical Pribilof Islands area blue king crab estimated mature male biomass (black, line) and fishery catch (red, open circles) from 1973 to 2015 (NPFMC 2015).

The Pribilof Islands blue king crab, Paralithodes platypus, stock is currently the only overfished stock in the North Pacific (NFMS 2015). This Bering Sea fishery began in 1973 with landings peaking around 11 million pounds in the 1980/81 season. From the mid-1980s through early 1990s, harvests decreased to less than 1 million pounds (Figure 1). To protect this declining stock the North Pacific Fishery Management Council (NPFMC) established the Pribilof Islands Habitat Conservation Zone (PIHCZ, Figure 2a) in 1995, where use of trawl gear is prohibited (NPFMC 2015). Following 1995, continued declines in the blue king crab fishery resulted in a closure from 1999 to present. The Pribilof blue king crab stock was declared overfished in 2002, and a rebuilding plan was established in 2003 (NPFMC 2014). A lack of stock recovery led to inadequate progress toward rebuilding the stock within the initial 10 year rebuilding period. In response, a new rebuilding plan was adopted by the NPFMC in 2012 and approved by the Secretary of Commerce in 2015 that closes the PIHCZ to pot fishing for Pacific cod. The fishery closure in addition to habitat protection and bycatch reduction measures have so far 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 to the fishery, and indeed even the most optimistic models do not show the stock attaining BMSY before ~50 years (NPFMC 2014). Our project will investigate if juvenile recruitment limitation or a bottleneck in juvenile stages are occurring to inform potential future rebuilding efforts.

Figure 2. a) Pribilof Islands Habitat Conservation Zone. Trawl fishing has been prohibited year-round in this zone since 1995, and as of 2015 Pacific cod pot fishing is prohibited as well. b) Distribution of blue king crab in Alaska (NPFMC 2015).

Figure 2. a) Pribilof Islands Habitat Conservation Zone. Trawl fishing has been prohibited year-round in this zone since 1995, and as of 2015 Pacific cod pot fishing is prohibited as well. b) Distribution of blue king crab in Alaska (NPFMC 2015).

Understanding why the blue king crab stock has declined and is not recovering has ramifications beyond the directed blue king crab fishery, because management measures for rebuilding of the Pribilof Islands blue king crab stock have decreased both opportunity and harvest in other fisheries. The Pribilof Islands red king crab fishery was closed in 1999 and has not reopened because of concerns for blue king crab bycatch (NPFMC 2015). The PIHCZ excludes trawl and now pot fisheries, with direct implications for Pacific cod (Gadus macrocephalus), yellowfin sole (Limanda aspera), rock sole (Lepidopsetta polyxystra), and flathead sole (Hippoglossoides elassodon). Other fisheries are likely affected as well, including walleye pollock (Gadus chalcogramma), arrowtooth flounder (Atheresthes stomias), sablefish (Anoplopoma fimbria), halibut (Hippoglossus stenolepis), and Greenland turbot (Reinhardtius hippoglossoides). Local residents in the Pribilofs are affected, because the CDQ groups own blue and red king crab and groundfish quotas.

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.

Figure 3.  Distribution of juvenile blue king crab (carapace length > 20 mm) and complex habitat types around St. Paul Island in the early 1980s (data from Armstrong et al. 1987).  The size of blue circles indicates the number of blue king crabs caught in a 5 minute rock dredge tow; Xs indicate stations where no crabs were caught.  Blue and purple shading indicate the extent of ‘rock and gravel’ and ‘shellhash’ habitats respectively.  Note that the habitats can and do overlap.  Green lines are isobaths with the depth indicated in meters. (Map courtesy of Chris Long, NOAA)

Figure 3. Distribution of juvenile blue king crab (carapace length > 20 mm) and complex habitat types around St. Paul Island in the early 1980s (data from Armstrong et al. 1987). The size of blue circles indicates the number of blue king crabs caught in a 5 minute rock dredge tow; Xs indicate stations where no crabs were caught. Blue and purple shading indicate the extent of ‘rock and gravel’ and ‘shellhash’ habitats respectively. Note that the habitats can and do overlap. Green lines are isobaths with the depth indicated in meters.

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.

Regime shift
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.

Figure 4.Year-0 red (Paralithodes camtschaticus, left) and blue (Paralithodes platypus, right) king crabs. Morphological differences between species occur in the early benthic phase, including larger size, more pronounced spines, and monochromatic coloration of red king crab. Photos by W.C. Long and S. B. Van Sant. (Daly and Long 2014a).

Figure 4.Year-0 red (Paralithodes camtschaticus, left) and blue (Paralithodes platypus, right) king crabs. Morphological differences between species occur in the early benthic phase, including larger size, more pronounced spines, and monochromatic coloration of red king crab. Photos by W.C. Long and S. B. Van Sant. (Daly and Long 2014a).


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.