Most British Columbia herring begin sexual maturation in the late summer and become sexually mature in the subsequent March or April. As they mature, most stocks migrate from summer feeding grounds to overwintering areas and then to shallow nearshore spawning areas. Also, newly recruited, sexually maturing fish join the adult spawning stocks, either on the summer or winter grounds. These events occur in an unpredictable environment. Consequently, reproduction in Pacific herring (Clupea harengus pallasi) may be viewed as a biological problem of maintaining synchrony and precision: specifically the synchronous maturation of (1) males and females and (2) the recruit spawners with the adult fish that have spawned in previous years. Biological precision is required to ensure that gametes are released at the correct place at the correct time. Further precision is required to achieve an appropriate balance between egg number and egg size, and then to control the density of eggs deposited on the bottom. The annual temperature regime is a vital regulator of these processes. Generally, the warmer the temperature, or lower the latitude, the earlier the maturation and spawning time. There are some notable exceptions that indicate local adaptation to other environmental cues. Also, experimental work indicates that food supply can affect the maturation rate. Intensities of egg depositions are generally similar among Pacific North American stocks, and indeed for many documented herring spawnings from Asian and European stocks. It has been shown that eggs in the middle of very thick spawns have lower survival so there is a selective advantage for biological mechanisms that ensure more even and lighter egg densities. This paper suggests that egg density in Pacific herring is controlled by a biological feedback mechanism involving milt concentration and hydrographic factors. Earlier spawners tend to be larger and there is a positive relationship between female size and egg size, a tendency consistent with other herring stocks and species. Size-adjusted fecundity, when compared among different regions and years, is strikingly uniform within British Columbia. On a broad geographic scale (California to Alaska), size-specific fecundity declines with latitude. Recent experimental work reveals that total egg number is higher at early stages of maturation and decreases as spawning time approaches. Presumably, this decrease reflects selective resorption of some developing oocytes, and probably occurs in response to available body energy. The duration and geographical variation in spawning time is relevant to questions concerning the evolutionary and taxonomic relationships between Pacific herring and Atlantic herring (Clupea harengus harengus). Other aspects of reproductive biology have important management implications, particularly for (1) predicting recruitment, (2) deriving stock estimates from spawn surveys, and (3) managing fisheries.
Advection, diffusion and mortality of Pacific herring larvae Clupea harengus pallasi in Bamfield Inlet, British Columbia