Jamestown Studying Spot Shrimp Fecundity

BLYN (June 18, 2007) – Aleta Erickson gently tweezes 100 eggs, each the size of a large grain of sand, from the body of a pregnant female spot shrimp. Taking the time to count shrimp eggs – a female can carry as many as 5,000 – is giving the Jamestown S’Klallam Tribe important information about the spot shrimp population in the Strait of Juan de Fuca.

Erickson, the tribe’s marine ecologist, and Lohna O’Rourke, the tribe’s biologist, have been looking at shrimp fecundity, a measurement of egg production, during the year-long study. As part of this study, females with eggs are closely measured and the total number of eggs are counted – a method that, while time consuming, is believed to be the most accurate method of determining fecundity. In a traditional fecundity study, a representative sample of the eggs is counted and weighed to determine an average number of eggs produced. Both methods are being evaluated to determine the extent of the differences between methods.

“An important goal of shellfish management is maintaining the population at an adequate level to ensure availability for future harvest,” said Kelly Toy, Jamestown S’Klallam Tribe’s shellfish manager. “To measure the effects to the harvestable population, managers need to know to what extent fishing reduces the egg production of a stock. This requires estimates of many factors, including the average fecundity.”

The Washington Department of Fish and Wildlife has collected fecundity data only in Hood Canal, and not in other areas of Puget Sound. The spot shrimp in the Straits are significantly larger than Hood Canal spot shrimp and produce more eggs. The number of eggs a female carries has been shown to be related to the size of the shrimp. Although the areas are geographically different, it appears that the difference in fecundity is related to size and not environmental factors.

Adding to the complex situation, shrimp begin their lives as males and for a short time period move into a transition stage, and then become females. Increased fishing pressure may have negative impacts to the shrimp populations by decreasing the size when transition occurs, which in turn will affect the number of eggs produced. Baseline data is needed to be able to monitor changes in size and fecundity.

Spot shrimp are a sensitive species and too much fishing has led to excessive harvesting that gradually depleted the population on Washington’s coastal shrimp beds. Spot shrimp harvest quotas in Puget Sound are based on historical catch rates and effort.

“We can not determine what the sustainable harvest quota is pre-season, so our hope is that this study will fill in one gap in shrimp management data,” Toy said. “It would be helpful to develop a recruitment model that managers could use to determine sustainable quotas. Currently, we are locked into these quotas that do not take into account high or low years of abundance.”

Spot Shrimp Fast Facts:

– Scientific name: Pandalus platyceros

– Life Span: 4 to 7 years (the further north they are, the longer the lifespan)

– Spot shrimp are found in the northeast Pacific Ocean from Southern Alaska to southern California, as well as in and around the Sea of Japan

– They are the largest shrimp in the North Pacific

– They are named for the white spots behind the head and in front of the tail

– Protandric Hermaphrodites: Shrimp are the few animals that spend their lives partially at males and later transition into females