Sardine Assessment Shows Cyclic Decline in Population
Pacific sardines are known for wide swings in their population: the small, highly productive species multiplies quickly in good conditions and can decline sharply at other times, even in the absence of fishing. Scientists have worked for decades to understand those swings, including a decline in the last few years that led to the Pacific Fishery Management Council‘s 
recommendation on April 13th to suspend commercial sardine fishing off the West Coast for the first time in decades..
An updated stock assessment by NOAA Fisheries’ Southwest Fisheries Science Center (SWFSC) was the basis for the Council’s action. Stock assessments are research tools that estimate the status and size of the sardine population. The Council uses the assessments to set fishing quotas.
Models that support the sardine assessment combine NOAA data on past and current abundance of sardine eggs, larvae and mature fish with other data on sardine biology and fishery catches. The data on sardine abundance come from two SWFSC research vessel surveys conducted off the West Coast each year.
These surveys employ two methods to estimate the current size of the sardine population. They use underwater acoustic equipment (like sonar) to estimate the size of fish schools, followed by the use of trawl nets to verify the species comprising the schools. Additionally, the surveys employ devices that measure the density of sardine eggs in the water as a gauge of sardine spawning. Scientists can then calculate how large the spawning population must be to produce the measured density of sardine eggs.
These data feed a computer model to estimate sardine population trends and provide the foundation for projections of the total population of sardines off the West Coast in the next fishing year.
“The assessment produced this year suggests that cool ocean water temperatures off the West Coast beginning around 2007 may have reduced the survival of juvenile sardine resulting in a population decline”, said Kevin Hill, a fisheries biologist who oversees the stock assessment for the SWFSC. The number of surviving young fish appears to have dropped to the lowest levels in recent history and has likely remained low in 2014. This has led to a steady decline in the fishable sardine stock biomass, which is defined as the total volume of sardines at least one year old. This is the measure the Council relies on when setting fishing quotas.
“The environment is a very strong driver of stock productivity. If ocean conditions are not favorable, there may be successful spawning, but fewer young fish survive to actually join the population,” Hill said. “Small pelagic fish like sardine and anchovy undergo large natural fluctuations even in the absence of fishing. You can have the best harvest controls in the world but you’re not going to prevent the population from declining when ocean conditions change in an unfavorable way.”
The current decline adds to a series of ups and downs that illustrate the boom-and-bust nature of sardine populations. The sardine biomass rose from about 300,000 metric tons in 2004 to a high point of more than 1 million in 2008 and is predicted to decrease to an estimated 97,000 metric tons by this coming July.
Because of these swings in sardine populations, the Council’s management framework for sardines includes built-in mitigation measures and safeguards to exponentially reduce fishing pressure as the stock declines. One of these Council measures is a cessation in directed fishing on sardines when the biomass falls below 150,000 metric tons. “The fishing cutoff point is included in the guidelines adopted by the Council and is designed to maintain a stable core population of sardines that can jump-start a new cycle of population growth when oceanic conditions turn around,” Hill said.
In the course of reviewing the 2015 updated assessment, it became evident that the final model used in the 2014 assessment did not correspond to the best fit to the data. The data were reanalyzed and a better fit to the 2014 model was achieved. This re-examination resulted in a lower 2014 biomass estimate of 275,705 metric tons, down from the previous estimate of 369,506 metric tons, which is still above the fishing cutoff value of 150,000 metric tons.
The revised model applied to the 2015 assessment resulted in a biomass estimate of 97,000 metric tons, which is below the fishing cutoff. As a result, the Council decided to close the 2015-2016 sardine fishing season and requested that NOAA Fisheries close the remainder of the 2014-2015 sardine fishing season. The sardine population is presently not overfished and overfishing is not occurring; however, the continued lack of recruitment observed in the past few years could decrease the population, even without fishing pressure.
The NOAA Ship Bell M. Shimada is currently conducting a new sardine survey off the West Coast to collect updated information on the size and location of the sardine stock. In addition, a large-scale 80-day survey this summer will collect data on sardine and whiting (hake) populations from the Mexican border to Canada. This new information will support the next stock assessment SWFSC prepares for the Council and NOAA fisheries managers.
Learn more:
Pacific sardine stock assessment
Executive summary
Full report
In the Field: Spring Sardine Survey 2015
Pacific Fishery Management Council Coastal Pelagic Species
California Cooperative Oceanic Fisheries Investigations (CalCOFI)
Video – Coastwide Sardine Survey
Green Seas Blue Seas – Interactive Guide to the California Current
For more information, please contact: Michael.Milstein@noaa.gov or Jim.Milbury@Noaa.gov (West Coast Regional Office Public Affairs), Dale.Sweetnam@noaa.gov (Southwest Fisheries Science Center) and Joshua.Lindsay@noaa.gov (West Coast Regional Office)
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