A new study found that releasing red king crabs as early as possible after they are reared in a hatchery may improve young crab survival and save operational costs. Researchers at the Alaska Fisheries Science Center have noted that the optimal time to release hatchery-raised red king crabs is immediately following their transition from freely swimming planktonic larvae to settling as bottom-dwelling juveniles.

The red king crab was one of Alaska's most important commercial and subsistence fisheries. In the 1960s, it was especially commercially important around Kodiak. However, the stock crashed in the late 1970s. Researchers believe the crash was a combination of climatic shifts, changes in the food web structure, recruitment failure, and overfishing.

According to NOAA Fisheries and the Alaska Fisheries Science Center, the commercial fishery has been closed since 1983, and the Kodiak stock still has not recovered. Due to the lack of recovery, the consideration of stock enhancements has grown through the release of hatchery-reared juveniles to bolster the wild population.

The Alaska King Crabs Research Rehabilitation and Biology program (AKCRRAB) was formed by NOAA Fisheries, commercial hatcheries and fishing groups, university groups, and State and Tribal governments. As an Alaska Sea Grant partnership and conducted by a research program coalition of state, federal, and stakeholder groups' views to examine the region’s long-term economic development and sustainability.

AKCRRAB's mission is to understand the large-scale culturing needs of wild red and blue king crab stocks and to perfect strategies for hatching and rearing king crabs to a stage where they can be released into the wild and contribute to reversing low wild stock abundance in AK. They believe that gaining this knowledge base will help policymakers make informed decisions about whether to one day pursue active rehabilitation of the depressed wild king crab stocks through hatchery enhancements.  

Preliminary work has laid the foundation for the experimental releases of red king crabs raised in captivity, including:

• Optimizing hatchery techniques for rearing red king crab
• Field and laboratory experiments on red king crab behavior
• Feeding, predation, and field surveys to determine optimal habitat for a release area

“The next step to effective stock enhancement is developing strategies that maximize post-release survival,” said Chris Long, the lead author and research fishery biologist at the Kodiak Laboratory.

During a set of experiments before this work, Long and his colleagues evaluated the number or density of where the hatchery-reared red king crabs were released and how it would affect their survival. 

“We had to figure out if timing and size at release would make a difference.” Long added.

During the experimental study, the hatchery-reared red king crab juveniles were released about every six weeks throughout the summer, beginning in June. Scientists recorded how many crabs died or moved out of the release area for six months. They also recorded how many predators were in the area and what kinds of predators were present. They found that the juveniles with the most significant mortality rates were during the early summer, and that’s believed to be due to their size.

The extended hatchery rearing period for the later release of larger crabs presented challenges such as high cannibalism rates in the hatchery communal holding conditions. Long shared, “Rearing individual crabs in isolation improves survival, but is labor intensive on a large scale, affects growth, and could impede brain development. Stock enhancement programs must balance these trade-offs to maximize overall success.”

Based on the overall findings from the study, researchers recommend that future work should focus on methods to increase post-release survival, especially during the first 24 hours and during the first two to three weeks. Research also suggests releasing the juveniles under screens or cages to provide temporary protection while they adapt to the environment. They also mentioned releasing the crabs at night when predation is lower.

“It also would be worthwhile to experiment with some techniques to condition hatchery-reared crabs to predators,” shared Long.

There was evidence that exposure to predators before release can induce hiding behaviors that would make red king crabs more challenging to detect. Researchers believe that this may decrease predation rates quickly after release.