Successful year classes of pollock are largely determined by survival of early life stages. (Photo: NOAA)

Climate change and fishing could change Alaska pollock spawning time

UNITED STATES
Saturday, November 17, 2018, 03:00 (GMT + 9)

A team of researchers determined that spawning time of Alaska pollock varied by as much as three weeks over the past three decades in the Gulf of Alaska and attribute these changes to the effect of climate and fishing. 

These scientists explain that if the resource arrives too early, there may not be enough food and that if they arrive too late, the young fish will have less time to grow and will be small compared to their predators and competitors.

In addition, 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--how many fish are available to the fishery two or three years later.

“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,” explains 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 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,”

Rogers pointed out.

As the age of the spawning population increases, spawning begins earlier (a). Warmer temperatures mean earlier spawning to a point around 4 ℃; above that temperature, spawning time levels out (b).(Photo: NOAA)

Using these resources, her team 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. 

NOAA Fisheries scientists collecting larval fish and their plankton prey during a Gulf of Alaska Survey. (Photo: NOAA) ►

Roger’s team highlighted that 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. 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 7 days later and shorten the spawning season by 9 days compared to an unfished population, independent of climate conditions,” stated Rogers.

That shift could cause young fish to arrive out of sync with their food in two ways: by decoupling the arrival of first feeding fish larvae from temperature-driven changes in plankton production; and by reducing the window over which young fish are delivered into the ecosystem, thus increasing the risk of mismatch with plankton production.

Rogers 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,” the researcher concludes.
 
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