Research Document - 1999/014
Distribution and timing of herring spawning in British Columbia
By D.E. Hay and P.B. McCarter
This paper examines spatial and temporal variation in herring spawn in British Columbia, from 1928 to 1998. We present summaries of temporal variation in indexes of spawn abundance for 6 different regions: (1) Queen Charlotte Islands (QCI), (2) Prince Rupert District (PRD), (3) Central Coast (CC), (4) Johnstone Strait (JS), (5) Strait of Georgia (SOG), and (6) West Coast of Vancouver Island (WCVI). Within the regions we also distinguish between 'non-assessment' areas and 'assessment' areas. Assessment areas are the geographic units applicable to the annual stock assessments, and they make up approximately 55% of the total coast. In all regions, the total amount of spawn has fluctuated during the last 71 years, but the trend varies with location. The trend in spawn deposition for the last 20 years has been to increase in 3 of the 5 assessment regions: PRD, CC and SOG. No trend is clear for WCVI but there is a long-term decline in QCI, although QCI spawn indexes have increased in the last few years. There appears to be a decline in spawn indexes for most non-assessment areas in the QCI, CC, and SOG. Spawn has decreased in the JS, a non-assessment region, but increased slightly in the non-assessment areas of WCVI.
In general, the geographical range of spawning areas is contracting. The BC coast consists of 108 different geographic units called herring sections, 101 of which have been used for spawning in one or more years since 1928. The numbers of herring sections receiving spawn is lower in 1998 than in all previous records since the 1930s, when the records were known to be incomplete. In part, however, this recent reduction could reflect declining survey effort in recent years. Similarly, the duration of the spawning period is becoming shorter, with later starts and early completions, in most areas. Some but not all of this trend also could be attributed to the reduction in survey effort. In the 3 major areas where spawn indexes are increasing, they are increasing substantially, so that the total coastal spawn deposition, if measured by the indexes presented here, is higher than in all previous assessment years. Therefore, as a generalization, we observe that spawning is contracting in space and time but increasing in abundance. The recent trend for an increase in spawn deposition is consistent with trends in spawning biomass reported in recent assessment documents, but the assessment documents do not consider the declining spatial and temporal ranges of herring, or make any comment about spawn in non-assessment areas. The reasons for the spatial and temporal changes in spawn deposition are not clear, but could be related to one or more factors including fisheries and climate change. Unfortunately some of the apparent changes could reflect declining survey efforts, particularly in the non-assessment areas.
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