Pacific sardine (Sardinops sagax) COSEWIC assessment and status report: chapter 6

Biology

General

There are presently two main spawning areas off southern California and Baja California. The major and northern spawning ground is between Point Conception and Ensenada (Figure 2). It is about 400 km long and extends up to 325 km offshore (Culley, 1971, Marr 1960). The other main spawning area, about half the size, is off central Baja California. In addition, there is an inshore fall spawning area off central Baja and a winter-early spring spawning area within the Gulf of California (Marr 1960). In the southern California offshore area, spawning occurs between April and May at temperatures of 13° to 16.5° C. In the lower California area, spawning is from March to April at similar temperatures. The fall spawning in this area occurs from August to September at temperatures of about 18° to 23° C. The Gulf of California fish spawn from February to March.

The evidence for sardine spawning in Canadian waters is largely circumstantial. Hart (1973) reported that sardines with ripe eggs have been found in Canadian waters, but no spawning was known to occur here. Williamson (1930) noted that in June and July some schools of ripening sardine were taken off the west coast of Vancouver Island. He also reports that “at different times, but not every year, small-size pilchards (3 inches long) have been reported in Clayoquot Sound, Hesquiat and Malksope Inlet, and they are recorded as having been taken at Nootka this year”. More recently, Ware (1999) reported evidence of eggs and larvae being taken in Canadian waters during 1992 and 1993 and again in 1997, all unusually warm years. McFarlane and Beamish (2001) also reported that ripe females were collected on the west coast of Vancouver Island in July 1997 and age 0 juveniles (mean=10 mm) were collected the following March and April, suggesting successful spawning in the area. It is known that sardines have spawned off the coast of Oregon just to the south in recent years (Bentley et al. 1996). Thus, it appears that in warm periods, such as during strong El Niño events, environmental conditions may be conducive to sardine spawning in Canadian waters. However, this does not appear to be an annual event, thus it is unlikely that a self-reproducing population exists in these waters.

Pacific sardines are batch spawners; large fish (21 cm) release 30-65,000 eggs per spawning (Hart 1973). A single large female can spawn about three batches, releasing almost 200 000 eggs per spawning season. Small fish (13-15 cm) appear to spawn about 30 000 eggs per season. Most spawning occurs during the first part of the night. Spawning behaviour differs from the usual schooling and consists of fish darting about and leaping out of the water (Culley 1971). The eggs are about 1.6 mm in diameter and take two to four days to hatch at 16° to 14° C. The eggs are deposited and fertilized in mid-water, and remain pelagic until hatching. Most of the eggs are found in the upper 25 m of the water column (Culley 1971). The larvae are about 3.5 mm in length and resorb the yolk sac after four to seven days. By the end of two to three months they are about 34 mm in length, and by the end of the first year they reach 115 mm. The length is about 31 cm for a fish 10 to 12 years of age. Females grow faster and larger than males (Culley 1971). The instantaneous natural mortality rate has been estimated from age composition and tagging information at about 0.40 (Murphy 1966). The age of maturity is variable and appears to be a function of stock biomass. In large stocks, only some of the two-year-olds are mature, while in small stocks all of the two-year-olds appear to mature (MacCall 1979). The young sardines move inshore as they grow and congregate in schools near beaches. Each year, beginning in their second summer, the fish migrate northwards early in summer and travel south again in the fall. Hart (1938, 1973) suggests that with increasing age the migration becomes farther with the oldest fish being found farthest to the north. The migrations appear to be complex, with timing and extent of movements being affected by oceanographic factors.

Physiology

Pacific sardine have been reported to over-winter in inlets of the British Columbia coast. Co-incidentally, there have been reports of mass mortalities (Hart 1943b). Foerster (1941) speculated that this was caused by a dietary deficiency. However, Hart (1943b) noted that the mortalities tended to occur at the heads of inlets where fresh water inflow could result in reduced water temperature. He also suggested that lack of plankton availability could be responsible for these mortalities. Hart’s (1943b) description is similar to recent observations in the Central Coast and Queen Charlotte Strait areas of British Columbia where sardines succumbed to VHS (viral hemorrhagic septicemia). From November 1998 to February 1999, large numbers of sardines were reported dying at numerous locations in the Queen Charlotte Strait. Samples collected from Beaver Cove were found to be infected with VHS virus. The strain of VHS virus appears to be identical to that commonly observed in Pacific herring (Clupea pallasi) from the British Columbia coast (personal communication - G. Traxler, Fisheries and Oceans Canada, Pacific Biological Station, Nanaimo, B.C. 14 November 2001). VHS has also been found in sardines off the California coast but it is unknown whether this is the identical strain to that found in the B.C. samples.

The mass die-offs occurred during strong El-Niño events and warmer water conditions, which appear to favour greater sardine migration into Canadian waters (Ware 2001). While the proximate cause of death of these sardines was probably VHS infection, it seems likely that stress from reduced water temperature and possibly from food limitation triggered the disease outbreak. Water temperatures observed during an earlier die-off in the Central Coast (Smith Inlet) in February, 1998 were measured at 7° and 8.5° C (personal communication - D. Kieser, Fisheries and Oceans Canada, Pacific Biological Station, Nanaimo, B.C. 10 March, 1998 ). Ahlstrom (1960) reported that the lower temperature at which sardine can live is approximately 7° C.

Movements/dispersal

Tagging of sardines in British Columbia waters and off California demonstrated the annual migration patterns of sardines to be northward in the spring and back to California in the late fall (Hart 1943a, Janssen 1948). Recent analysis of historical fisheries shows that rate of movement into Canadian waters is influenced by water temperatures during the spring and early summer (Ware, 2001). The extent and speed of the annual northward migration of sardines is directly related to the location of the 12° C isotherm. As a result, in warm El Niño years there are more extensive migrations into Canadian waters. Other factors, such as local environmental conditions and food availability, also play roles in the dispersal of sardines. Estimates of the proportion of the total annual Pacific sardine catch that were taken in Canadian waters, from 1917-1948, suggest that on average about 10% of the sardine population migrates into British Columbia (Ware, 1999, 2001).

Nutrition and interspecific interactions

Pacific sardine feed extensively on copepods while young and switch to diatoms as they mature (Hart 1938, 1973; McFarlane and Beamish 2001). Ahlstrom (1960) noted that sardines are opportunistic and consume fish larvae and other zooplankton when available. Sardines are prey to a wide variety of predators including commercially important fish species, seabirds, and marine mammals (Bargmann 1998, Culley 1971, Ahlstrom 1960). Pritchard and Tester (1944) reported that sardines were the dominant prey item for chinook (Oncorhyncus tshawytscha) and coho (O. kisutch) salmon during the 1930s when they were abundant off the British Columbia coast. It is expected that if the sardine population continues to increase, they will again become an important food item for salmonids in the Pacific northwest.

The dramatic collapse and disappearance of the sardine from the west coast of North America during the mid-20th century stimulated research into a better understanding of the dynamics of this and other pelagic species that exhibit large fluctuations in abundance. Studies of sediment cores in the Santa Barbara Basin (California) have used fish scale deposition to reconstruct the relative abundance of pelagic species for the past two millenia. These data reveal cycles of approximately 60 years’ duration in the abundance of both sardine and northern anchovy (Engraulis mordax); moreover, during sardine absence, anchovy tend to be more abundant and vice versa (Soutar and Isaacs 1969,1974). Because the negative associations are only weakly correlated, the mechanism driving the fluctuations remains unclear. There may be a parallel response to large-scale environmental change or competition for food or other biological interaction (Baumgartner et al. 1992). Baumgartner et al. (1992) found nine major recoveries and nine subsequent collapses of the sardine population in the past 1700 years. The recoveries range from 20 to 70 years, with an average of 36 years, while the collapses range from 20 to 50 years, averaging 30 years in length. Thus, the recent collapse and recovery of sardines in the California Current system appears to be a recurring biological phenomenon that may be mediated by both climatic factors and biological interactions, and can be expected to recur in the future despite human intervention.

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