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FEATURE ARTICLE - Shellfish: Resources and invaders of the north

Our latest feature explores the dynamics of snow crab in the eastern Bering Sea and US portions of the Chukchi and Beaufort seas under climate change.
Published: 2 April 2020

​Gordon H. Kruse (College of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Juneau, Alaska, U.S.A.​) ​provided the keynote at ICES symposium ​​​Shellfish - Resources and ​Invaders of the North. 

The goal of the recent ICES symposium ​​​Shellfish - Resources and ​Invaders of the North was to discuss the role of cold-water shellfish as a harvestable resource and as important ecosystem players in northern hemisphere marine ecosystems. The symposium focused on four key questions: 

  1. How do we exploit them sustainably?
  2. Can we explain recent changes in distribution and population dynamics, and what should be the management responses?
  3. What are the ecological effects of invasive species, and should they be controlled by excessive harvests or managed for sustainability?
  4. The ecosystem effects of the boom and bust of large shellfish populations are potentially massive - can these impacts be quantified?

In our keynote address, my co-authors* and I attempted to begin to answer these questions by focusing on snow crab (Chionoecetes opilio) in the eastern Bering Sea (EBS) and United States' (US) portions of the Chukchi and Beaufort seas. Snow crab provides a good case study because the distribution of this Arctic species spans the North Atlantic and North Pacific Oceans, it is of keen interest (e.g., snow crab was addressed in 18 oral and 7 poster presentations at this symposium), and our understanding about snow crab in the North Pacific has recently advanced. In our review, we cover environmental and ecological processes that appear to determine the changing geographic distribution, abundance, and productivity of snow crab in this region.

Snow crab support a large commercial US fishery in the EBS with annual catches that have fluctuated between 86,000–1,491,000 t since 1982. They are sma​​​​ller in body size and less abundant in the US portions of the Chukchi and Beaufort seas, where an Arctic fishery management plan prohibits fishing owing to ecosystem considerations and scientific uncertainty. In recent decades, these regions have undergone large interannual changes in temperature and sea ice extent with major ecological consequences. For example, extraordinary weather during the winter of 2017/2018 resulted in the lowest ice year ever recorded for the EBS, as well as the highly unusual lack of a cold pool of bottom water (<2C) that had previously formed annually as a “footprint" of winter sea ice extent. Recent very warm conditions resulted in delayed phytoplankton blooms, few large copepods and euphausiids, and die-offs of seabirds, ice seals, and gray whales.

Associated with warming and reduced sea ice, the southern edge of EBS snow crab distribution shifted 230 km to the northwest during 1982 – 2006. Cold pool extent explains 57% of the variance in commercial snow crab catch in this region. In 2019 bottom trawl surveys revealed that the majority of mature male snow crab occur in the northern Bering Sea, outside of the EBS fishery management area covered by standard surveys. Based on recent surveys of the US Arctic, our estimates of snow crab unfished biomass were twice as large for the Beaufort Sea and an order of magnitude larger for the Chukchi Sea than estimated in the US Arctic Fishery Management Plan in 2009. However, conditions for growth remain suboptimal, as 0% (Chukchi) to 25% (Beaufort) of the unfished biomass is comprised of crab of harvestable size. An individual-based model using a regional ocean modeling system revealed that the US Arctic is a sink for snow crab larvae from the EBS. However, we also estimated that that locally sourced recruitment could account for the full abundance of snow crab in the Chukchi Sea. The metapoulation structure of snow crab in this region remains to be fully resolved.

Changing thermal environments challenge snow crab reproductive biology. Ocean currents carry larvae to the northeast, where they recruit to the benthos. Crab migrate with ontogeny in a southwesterly direction across a broad continental shelf. Owing to sexual dimorphism in age- and size-at-maturity, females mature in the middle domain of the EBS in advance of males. In subsequent years, males mature and arrive in the middle domain, creating large swings in operational sex ratios. Female snow crab possess spermathecae, allowing sperm to be stored for fertilization of subsequent egg clutches. After mating, males continue to migrate to the outer domain, where they are targeted by a commercial fishery. Snow crab in the EBS have the lowest spermathecal loads of any snow crab population worldwide likely due to highly variable distributions of males and females, driven by ontogeny and a very variable thermal environment. Thus, EBS females are dependent on current-year mating rather than stored sperm for fertilization of egg clutches. The northward shift in the female distribution contributes to lower reproductive success as female body size declines with increasing latitude, and lower spermathecal loads are associated with smaller body size. Moreover, just a few female age classes contribute substantively to reproduction. The female molt to maturity is her final life-time molt.  Thereafter, with increasing age, fecundity at size declines, variability in fecundity at size increases, and senescence becomes common. Interestingly, female snow crab have the capacity to switch between annual (>1C) and biennial reproductive cycles (<1C). So, while same-sized females reared in the cold north produce about half of the number of egg clutches per lifetime as those in the warmer south, increased temperatures may double reproductive output in the north.

This species is further challenged by predators, particularly Pacific cod (Gadus macrocephalus), which are major consumers of juveniles. The southerly distribution of EBS snow crabs appears constrained by cod predation in addition to ocean currents that carry larvae northwestward. However, with the loss of the cold pool in recent years, Pacific cod have greatly expanded their distributions to the northern Bering Sea, as well, where anecdotal reports indicate very high levels of crab predation. On a positive note, snow crab seem tolerant to ocean acidification levels projected over the next two centuries. However, ocean acidification effects on juveniles, a sensitive life stage for the congener C. bairdi, have yet to be studied.

​​​In summary, climate change appears to pose mostly negative effects on snow crab in the EBS, although warming conditions promote increased crab abundance in the Arctic. With ongoing climate changes, biological reference points need to be re-estimated for each stock component. Shifts in spatial distributions challenge our current understanding of stock units and the appropriateness of the long-standing survey design, both of which should be re-evaluated in a metapopulation context. A spatially explicit fishery management plan reflecting these changing dynamics will be required to assure the proper assessment and sustainable management of this valuable resource into the future.

 

*Co-authors: Gordon H. Kruse, Lauren M. Divine, Laura M. Slater, and Joel B. Webb​.


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​Gordon H. Kruse ​provided the keynote at the recent ICES symposium ​​​Shellfish - Resources and ​Invaders of the North.​

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FEATURE ARTICLE - Shellfish: Resources and invaders of the north

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