Determing the rate and causes of egg loss following spawn events
Because herring are mobile and aggregate, estimating their population size can be difficult. Consequently, herring eggs are used to back calculate the number of reproductive adults that spawned them.
However, there is often a loss or death of eggs between the time the eggs are deposited and the time they are counted in yearly dive surveys (1-17 days after spawning). Such egg loss prior to spawn surveys can result in underestimates of the actual number of eggs originally spawned, leading to underestimates of the abundance of the spawning population.
Measuring herring egg loss rates after spawning is critical because this rate is a key number in the current Department of Fisheries and Oceans (DFO) stock assessment model. To understand egg loss, Simon Fraser University Masters student Britt Keeling, under the direction of marine ecologists Margot Hessing-Lewis and Anne Salomon (Simon Fraser University, Hakai Beach Institute), conducted two linked studies.
In the spring of 2011, we conducted an experiment to determine the quantity and rate of herring roe consumption by marine predators. We based our experimental design on the traditional spawn-on-kelp (SOK) fishery by enclosing standardized pieces of spawn-on-kelp in predator exclusion cages that were suspended at different depths throughout the water. We then monitored egg abundance over a two-week period. [Keeling thesis ref here].
In 2012, we monitored herring egg loss following a spawn event for up to 22 days at 9 study sites within permanently placed quadrats. Divers returned to monitor the same quadrat every 3 to 5 days throughout the egg incubation period (18 to 22 days), visiting each site a total of 5 times. To determine potential herring egg predators, we also conducted dive surveys of fish and bottom-dwelling invertebrates at all survey locations.
Results:
1) Our 2012 observational dive surveys estimated egg loss rates from 59-75% over 6.8 days, which is the average lag time between spawn event and herring egg survey by DFO. This estimate of egg loss rate is more than double the value currently used to estimate herring spawning stock biomass for Canada’s Pacific herring fishery. This means that a large proportion of eggs could be uncounted, which could lead to uncertainty in current stock assessments.
2) Egg loss rates varied 4.5 fold among sites, and spawn area was found to be a key factor positively influencing egg loss rates. This high site-level variability is important to account for when estimating herring abundance and calculating uncertainty in stock assessments.
3) Experimental evidence from the 2011 study suggests that both egg predation and habitat type are also strong drivers of egg loss with the majority of eggs being consumed on the sea floor compared to those suspended in the water column
Experimental cage to test how spawn-on-kelp survives if major predators are kept out. These cages were placed at different depth: on the bottom, midwater and just below the surface.