CPR data proves essential in understanding North Pacific herring stock variability
Prince William Sound, in southcentral Alaska was once an internationally important herring (Clupea pallasii) fishery, however, since the Exxon Valdez oil spill in 1989, the population crashed and stocks have never fully recovered. The reasons behind this decline are still not clear, and disease, predation, and environmental variability may all play a part. Funding by the Exxon Valdez Oil Spill Trustee Council (EVOSTC) has supported scientific research since the environmental disaster, and over the last 4 years, SAHFOS has been involved with a long-term monitoring programme in the area, Gulf Watch Alaska. In collaboration with the EVOS TC funded Herring Research and Monitoring programme, the first paper to be produced between the two programmes has recently been published, and highlights the value of CPR data in understanding herring stock variability.
This research focuses on the relationships between first year growth of herring, sea temperature and their food (plankton). Measurements of herring scales were taken from 4-6 year old fish, to determine the amount of first year growth, as like rings of a tree, fish scales can reveal information on the history of the fish. These data were coupled with plankton data collected by the North Pacific CPR Survey from the adjacent Gulf of Alaska shelf and with mean monthly water temperatures calculated using data from the Cordova tide station in Prince William Sound.
Results found that overall, the year to year differences in first year growth of herring were more strongly related to plankton indices than to temperature. This relationship was only found between herring and the specific types of plankton herring feed on in their first year, such as diatoms and microzooplankton; during warmer years, the abundance, timing and quality of these important plankton taxa is greater. The study found that the strong relationship between diatom abundance and first year growth held even in the 2 years (2004 and 2005) that the growth-temperature relationship failed, suggesting that food quantity and/or quality are the strongest drivers of growth.
With sea temperatures predicted to increase in the future, changes in the planktonic prey-base of the herring is likely to occur, which as this study shows, has potential to adversely affect herring populations. Of course first year growth is only one factor in the success or otherwise of fish, however, this research highlights the value of long time series and represents a significant step forward in progressing our understanding of stock variability, particularly of commercially important fish species.
Batten, S.D., Moffitt, S., Pegau, W.S., and Campbell, R. (2016). Plankton indices explain interannual variability in Prince William Sound herring first year growth. Fisheries Oceanography, 25, 420-432. http://onlinelibrary.wiley.com/doi/10.1111/fog.12162/full