Research Document - 1999/177
Distribution of spawning eulachon stocks in the central coast of British Columbia as indicated by larval surveys
By P.B. McCarter and D.E. Hay
Abstract
The anadromous eulachon (Thaleichthys pacificus) spawns in the lower reaches of coastal rivers and streams from northern California to Alaska. Although the distribution and timing in some rivers is well known, the occurrence in other rivers is uncertain or unknown. The presence of larval eulachons in estuaries and marine waters adjacent to rivers is a strong indication that a river is used by eulachons for spawning. Some British Columbia rivers are known to have long-established runs, but the status of many other rivers is uncertain. In this report, the authors present data from larval surveys that confirm the presence of eulachons in central coast rivers where they were known or believed to occur. Several rivers that apparently support eulachons where they were previously undocumented were also identified.
Central British Columbia mainland inlets were surveyed in 1994, 1996 and 1997 to determine distribution patterns and relative abundance. A total of 767 plankton net hauls were completed in 3 intensive surveys. Salinity-temperature, depth profiles and bathymetric distributions of larvae were also examined in some inlets. Each survey was conducted in a two week period in the spring, after larvae had hatched and had been flushed from nearby eulachon spawning rivers into adjacent estuarine and marine waters. In most inlets that have two or more eulachon-spawning rivers, the geographical distribution of larvae in estuarine and marine waters was continuous, indicating that larvae from different rivers were mixed. In some instances, the larval distributions were continuous between adjacent inlets. The authors suggest that this apparent mixing of young larvae may limit or preclude the potential for differentiation of spawning populations between closely adjacent rivers, or inlets. In most inlets, it appears that estuarine circulation may retain larvae. This period of retention may last for a period of several weeks or longer. In some instances, our surveys detected larvae a month or more after hatching. In general, eulachon larvae were confined to the upper brackish outflow layer that extended out from some estuaries a distance of 100 kilometres or more. The authors looked for larvae in some small inlets where they had not previously been described, and sometimes we found some. The presence of larvae in these inlets, indicates that eulachon spawn in some nearby streams or rivers that had not previously been known to support eulachon spawning.
The authors discuss the results of these surveys in the context of the availability of suitable spawning rivers for eulachons and their present status. The authors estimate the eulachon spawning biomass required to produce the numbers of larvae they observed, but emphasize that these are not estimates of the total spawning biomass, which they believe would be larger, perhaps by an order of magnitude or more. They use these estimates, however, to provide an approximate biomass scaling among different areas and review these estimates in the context of the available information of eulachon spawning biomass estimate for different rivers. The report concludes with a brief discussion of the recent declines in eulachon populations, their present status and recommendations.
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