Interest in the mechanisms underlying variability in fish populations, dating back to Johan Hjort's seminal work in the early 20th century, is as strong as ever. The reason for this is that limited understanding of this changeability continues to restrict our ability to make management predictions. Moreover, there have been several recent observations that fishing increases population variability, and there is further worry that the aggregate variability over multiple stocks has been increased by diminished habitat diversity through the portfolio effect (in Pacific salmon especially). Underpinning all of these is the global concern that the physical drivers of variability may be altering due to a changing climate.
In this featured article, Botsford et al. show how environmental changeability in early survival affects the degree of population variability in different ways in three fish species with short, medium and long longevity: Coho salmon (~3y), Pacific Hake (~25y), and Pacific Ocean Perch (~90 y) respectively. The variability in each is the result of a recently-discovered phenomenon known as cohort resonance.
Cohort resonance consists of the greater sensitivity shown by fish populations to environmental variability on time scales near one generation time (average time span between birth of a fish and the birth of its offspring) as well as on very long time scales. This variability is shown to be greater in shorter-lived species. It increases with fishing (as with recruitment, egg production and catch) across all species, but more rapidly in longer-lived species. Cohort resonance is an additional plausible explanation for the observed increases in variability with fishing.
A better understanding of this phenomenon in age-structured upper trophic level species will be useful in combatting uncertainty in management, assessing population risk, evaluating the effects of habitat homogenization, and anticipating the effects of a changing climate.
Coho salmon (top) photo, Morgan Bond; Pacific hake (center), Tom Laidig; Pacific Ocean perch (bottom), John Butler