Critical Periods in the Marine Life History of Pacific Salmon

It is now exactly one century since the great Norwegian fisheries biologist Johan Hjort proposed his “critical period” hypothesis: That very early in the life history of marine fish such as cod, a single short period largely determined the major fluctuations in recruitment evident in fish populations (Hjort 1913). Hjort was an excellent scientist, and his original conjecture was that such a critical period might exist just after cod hatched from their eggs, when he viewed the larvae as particularly frail and thus especially vulnerable to environmental variability. However, Hjort also made clear in his writings that he viewed this hypothesis as merely a simple working theory and that research would have to move on to look at later phases in the life history if this initial simplifying theory was not supported. Unfortunately, such was rarely the case and many biologists came to state Hjort’s theory as a fact, and used it as a powerful rationale to justify focussing research efforts on the earliest life history phases, which were typically viewed as more tractable subjects for study than the later life history stages. For salmon populations, this typically meant proposing that the “salmon problem” (poor smolt to adult survival rates) was determined either in freshwater or in the first few days or weeks of life in the ocean. Although a second “critical period” has been proposed to occur in the first winter at sea (Beamish and Mahnken 2001) for salmon, the issue of whether specific critical periods actually exist (and thus make the recruitment problem potentially more tractable) has never been formally examined. After a century of research, there continues to be a persistent effort to simplify research on salmon by assuming that there is a critical period in the freshwater or early marine life history (and to propose large-scale research programs to study it) before actually establishing its existence. The consequences are important because, if the early marine phase is simply studied to assess the drivers of salmon returns without formally testing the theory that this life history period has the dominant role in determining recruitment, research programs may potentially continue for decades without ever establishing that they are studying the “wrong” period, wasting both intellectual and financial resources—and the opportunity to actually identify where the recruitment problem actually lies. The advent of prototype large-scale freshwater and marine telemetry systems, such as the POST array, now provides an opportunity to formally assess the critical period theory as it applies to Pacific salmon, as a large number of populations have now been tagged and their freshwater and marine survival followed for 1-2 months after release. A 10-fold decline in survival of many Pacific salmon populations occurred over the last three decades, with smolt-to-adult return rates dropping from >10% to ~1%. Using the POST telemetry array we tested whether mortality in the earliest life history phases is large enough to form the primary driver of adult recruitment rates, and whether the survival measured during the early marine period is now small enough to explain a ten-fold decline in smolt-to-adult recruitment.