NOAA study shows climate, fishing affect pollock spawn timing

A new study by NOAA shows both fishing and climate change have contributed to large variations in the spawning time of walleye pollock stocks in the Gulf of Alaska. 

An unprecedented data set spanning 32 years showed that spawn times swung up to three weeks for Alaska pollock, the largest fishery by volume in the United States. The study identified warming water and fishing impact as major contributors to these swings.

Warmer waters, NOAA researchers found, contribute to a longer spawning period that starts earlier. However, this only happens up to a certain threshold temperature at four degrees Celsius, according to Lauren Rogers, who headed the study. 

“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,” Rogers said.

The other main point of concern that arose from the study is that fishing weakens the population of older, larger female fish, which were found to be more robust in spawning, exhibiting earlier, longer spawn times.

“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.” Rogers said.  

The main problem with variation in pollock spawn time is that their spawning may not match up with the spring bloom of plankton, the principal food source for age-zero pollock. When fishing pushes the spawn date back and shortens the window, young pollock run a greater risk of missing the peak of the bloom. That means they may starve, or may be too small to compete for an already dwindling plankton supply at time when mortality is highest for pollock. 

Changes in spawn time that kill off large numbers of age-zero pollock are felt in fisheries two to three years down the line when pollock reach harvestable age. 

According to figures provided by NOAA, the five-year average catch for Alaska pollock was over three billion pounds, accounting for 55 percent of Alaska’s total catch by volume, and over 35 percent of the total catch nationwide. 

Photo courtesy of NOAA