A new NOAA Fisheries study on survival of young Alaska Pollock during the 2015 Blob year in the Gulf of Alaska includes significant factors were low-salinity conditions that affected egg buoyancy, low abundance of prey for larval fish and poor body condition of age-0 juveniles.
“Marine ecosystems are complex with lots of connections,” said fisheries scientist Lauren Rogers. “When you are trying to identify what might be behind a decline in abundance of fish at a particular age, it is often a series of factors that are at play.”
The absence of larval and juvenile fish was due to environmental conditions that were not ideal for Pollock growth and development during the heatwave, she said.
In the Gulf of Alaska, Pollock play a key role as both predator and prey and support a $100 million commercial fishery. To understand how that marine heatwave affected Pollock, Rogers and her colleagues used oceanographic and biological data compiled from years of research surveys. Their analysis considered the timing of spring phytoplankton bloom and the availability of zooplankton prey for larvae and juveniles. They also looked at how warmer temperatures and reduced prey quality affected the body condition and consumption demands of juvenile fish prior to their first winter.
They noted that the adult population of Pacific cod also experienced a significant decline, leading to a severe reduction in catch limits.
They also found that warmer temperatures, together with reduced quality of prey, increased the amount of food required for needed by nearly 20 percent compared to an average year.
Information generated through this research has already been used in making management decisions.
The study notes that the number of Pollock born in 2015 is the lowest on record. While it is not unusual to see variability in how many Pollock are born each year, the lack of new recruits from the Blob years, especially 2014 and 2015, has contributed to very low age diversity in the stock in recent years, the study said.