Where Have the Baby Fish Gone?
Fish larvae numbers have plummeted in the Strait of Georgia.
The early life of a fish is a dangerous time. Most fish will never make it to adulthood, succumbing instead to predators and competition. But those that do survive will form that year’s cohort of fish, going on to give rise to the next generation.
In the Strait of Georgia, larvae numbers of several fish species appear to have decreased over the last three decades. A new paper in the journal Progress in Oceanography shows that the concentration of larvae of some commercial species like Pacific hake and walleye pollock dropped between the early 1980s and late 2000s.
“The two most likely driving forces are fisheries and climate,” explains Brian Hunt, a biological oceanographer and Hakai researcher at the University of British Columbia who was a coauthor on the new paper.
By taking adult fish, fisheries decrease the overall population, in turn reducing the number of larvae in the water in a simple cause-and-effect relationship.
Effects from climate are more complicated. Northern smoothtongue, for example, is not fished commercially but declined just like hake and pollock, which are fished in BC. The authors cite changes in climatic conditions in the Strait of Georgia—“especially their interannual variations.” In other words, the climate has shifted, but also has become less predictable year to year, throwing off the timing of fish spawning events.
“The timing is quite critical,” says Hunt, referring in particular to the match between the timing of larval hatching and the timing of blooms in their plankton prey. Spawning too early can mean the larvae emerge into clear water without enough plankton to eat. Spawning too late can mean the spring bloom has passed, and the plankton are already too large to make good prey for the young fish.
No larvae, for any species, were shown to have increased between the two sampling periods, and the overall concentration of larvae dropped dramatically as well—a 75 percent decline. Not all fish did decline significantly, however; rockfish and herring larvae showed no significant difference between the 1980s and 2000s.
Hunt adds that the data do not necessarily show a long-term trend, nor do they tell the researchers definitively what could have caused the drop.
“It’s giving an indication of what might be important factors,” he says.
To really understand what caused the drop, and how it happened over time, Hunt would like to conduct a study that would span multiple years and seasons. Monitoring the fish continuously would help researchers like Hunt understand where the fish have gone—and maybe what can be done to keep them around.