Sea Watch: Study finds pollock spawning varies widelye

Federal fisheries managers are looking at a new study using an “unprecedented 32-year data series” to help understand what, if any, changes might be necessary to the opening dates of the pollock fishery in order to help ensure the survival of older, more successful spawning females.

The study showed that spawning time of pollock in the Gulf of Alaska varied by as much as three weeks over the past 30 years and found clear evidence that the changes were driven by both climate and fishing.

In a report from the National Oceanic and Atmospheric Association regarding the study, it was noted that changes in spawn timing have “major ecological and management implications.”

According to the study, an older spawning population started spawning earlier and over a longer duration than a population of predominantly young spawners.

“Timing is critical to survival of newly hatched fish as it determines the conditions they encounter,” the NOAA report states. “Many marine fish, like pollock, are adapted to spawn in time for offspring to meet the rapid increase of their plankton prey in spring. If they arrive too early, there may not be enough food; if they arrive too late, the young fish will have less time to grow and will be small compared to their predators and competitors.”

Because most mortality happens during the first few weeks of life for pollock, changes in spawn timing that affect larval survival can strongly affect recruitment success.

“To effectively monitor and manage pollock populations, managers need to understand what causes changes in spawn timing. With ongoing warming of the world’s oceans, we need to know how changing climate conditions interact with other processes, like harvesting, to influence spawning time,” said Lauren Rogers, the NOAA Fisheries biologist who led the study.

Toward that end, Rogers’ team investigated how pollock spawn timing has shifted over warm and cool periods and large shifts in age structure in the Gulf of Alaska.

The study produced two major findings.

The first was that warmer temperatures mean earlier and longer spawning periods, but only to a point. Above a threshold temperature, increased warming had no additional effect on spawn timing.

“Because temperatures are projected to be consistently above that threshold with ongoing ocean warming, our results suggest that pollock spawn timing will become more stable in the future,” said Rogers.

The second major finding was that older, larger females are spawning earlier and over a longer period, highlighting the importance of older mothers.

This is where fisheries management comes in: commercial harvesting leads to a younger, smaller population over time. In general, increased mortality reduces the mean age of a population, and this effect is strengthened if older individuals are targeted through size selective harvesting, common in the pollock fishery.

Besides direct effects of harvesting on age structure, fishing may cause evolutionary change by selecting for reproductive maturation at an earlier age or smaller size.

“Our models suggest that changes in pollock age structure associated with sustainable fishing can shift the mean spawning date to seven days later and shorten the spawning season by nine days compared to an unfished population, independent of climate conditions.” said Rogers.

That shift could cause pollock larvae to be out of sync with their food source due to temperature-driven changes in plankton production, as well as reducing the window over which young fish are delivered into the ecosystem, thus increasing the risk of mismatch with plankton production.

“Our models suggest that climate change will lead to an earlier, stabilized spawning season in the future.” Rogers said. “What we don’t know is how that will affect synchrony of first-feeding larvae with production of their zooplankton prey in spring.”

Rogers hopes future research will answer that question.

“We are looking at ways to evaluate match-mismatch with prey by comparing prey and larval fish production.” She also hopes to develop the model into a practical forecasting tool. “If we could use climate and age composition data to predict spawn timing 3-4 months ahead, the forecast could be used to make sure surveys are optimally timed to coincide with peak spawning periods.”

Rogers had high praise for all the effort that went into collecting the data used in the study.

“The strength of our study is comprehensive information from an amazing 32-year time series of larval fish size, age, and abundance, validated with maturation data from spawning females, and combined with at-sea process studies, laboratory experiments, and age readings. Using these resources, we were able to test for effects of climate and age structure on both mean spawn timing and duration, and forecast spawn timing under different scenarios of warming and fishing mortality,” she said.

Cristy Fry can be reached at

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