Evidence that the Migration of the Northern Subpopulation of Pacific Sardine (Sardinops sagax) off the West Coast of the United States Is Age-Based

Abstract

Analysis of fish movements has been an important area of study for fisheries ecology and population dynamics for decades. Pacific sardine, Sardinops sagax, along the west coast of the United States exhibit a well-defined large-scale seasonal migration. Larger and older fish are found in the northern reaches of their range during summer and contract to southerly offshore areas for spawning during spring. Because of the close correlation between fish size and age it has not yet been determined if movements are size- or age-based. Measuring spatial changes in the age structure conditioned on individual lengths was used to determine the roles of age versus length in the seasonal migration. S. sagax have a pattern of increasing age-at-length with seasonal northward movements and offshore movements for spawning. The pattern of increasing age-at-length with distance from the origin eliminates a solely length-based process of movement and supports age-based movement. Patterns in the size and age when fish first show migratory behaviors, coupled with the patterns observed during the spawning season, support a hypothesis that migratory behaviors are linked to age-based ontogenetic changes associated with maturation.

Use of parasites to clarify residency and migration patterns of Pacific sardine (Sardinops sagax) in the California Current

“…Other parasites recovered from Pacific sardine that contributed to differences among regions include the nematodes and the acanthocephalan that are both longlived taxa. For example, Anisakis nematodes, which mature in cetaceans, accumulate as long-lived larvae in fish intermediate hosts and have been successfully used as biological tags for many fish populations (MacKenzie, 2002;Mattiucci et al, 2008;Mattiucci and Nascetti, 2008). In our study, there was also a greater abundance of Anisakis spp.…”

“…Parasite life span and life history are important considerations for selecting parasite taxa as biological tags Abaunza, 1998, 2014). The parasite communities of Pacific sardine in this study included both long-lived nematodes and shorter-lived trematodes.…”

“…We found no significant differences between the parasite communities of Pacific sardine <200 mm SL and those of Pacific sardine 200-209 mm SL, indicating that there is not a clear cut off at 200 mm SL at which a Pacific sardine will migrate. A recent paper by McDaniel et al (2016) that included size and age data for Pacific sardine from British Columbia, Oregon and Washington, and Central and Southern California during 1981-2010 described a pattern of increasing age at length with distance from Southern California. They concluded that their results provide evidence that the migration of the northern subpopulation of the Pacific sardine to the northern reaches of their range in summer is age based, instead of length based.…”

“…One method that has successfully identified or confirmed migration patterns and stocks of marine fish species worldwide is the use of parasite species as biological tags (Thomas et al, 1996;MacKenzie and Abaunza, 1998;MacKenzie, 2002). Many parasite taxa can serve as biological tags because a fish can become infected with a parasite only within that parasite's endemic area (where all conditions for transmission exist).…”

“…Once in a fish host, parasites can be relatively long lived, remaining in or on the host from months to years, depending on the host-parasite association (Rohde, 1984). The technique of using parasites has limitations, such as time-consuming examination of hosts for internal parasites, a potential need for molecular techniques for correct identification, and a lack of adequate information on a parasite's ecology and biology (reviewed in Abaunza, 1998, 2014). Despite these limitations, this technique has often been used by scientific and fisheries agencies (Pita et al, 2016), and by 2014 more than 290 peer-reviewed manuscripts had been published on the topic (Timi and MacKenzie, 2015).…”