- The average size and age of chinook salmon (Oncorhynchus
tshawytscha) caught in commercial fisheries along the Pacific Coast
of North America have decreased substantially in this century. These
declines might be caused in part by changes in size and age at
maturity within the stocks contributing to those fisheries. Upriver
Brights (Brights), a stock of fall chinook salmon in the Columbia
River, are one of those stocks. The purposes of this study were to
(1) determine if average size and age at maturity of Brights have
declined, (2) gain a better understanding of the factors that may
contribute to such declines, and (3) describe potential consequences
of these changes.
Data from in-river fisheries suggest that the average weight of
mature Brights returning to the Columbia River has decreased
approximately 2.7 kg since the 1910s, an average rate of about 0.1
lb·yr⁻¹ (45 g·yr⁻¹ ). Most of the potential biases in these data tend
to make this estimate conservative. Insufficient data were available
to describe changes in average age at maturity.
There are many potential causes for the decline in average size
of mature Brights, including factors that affect very early life
stages. Other researchers have determined that size at maturity
appears to be highly influenced by inheritance, gender, and growth
rate. I describe how maternal size can influence -- through time of
spawning, choice of spawning site, and egg size -- the viability of
the young, which carry the dam's genes for size. The size-related
ability to produce viable offspring may have been changed by
modifications in the environment. Very little is known about how
changes in the natural environment for spawning, incubation, and
rearing may have contributed to a decline in average size at
maturity. Artificial propagation and rearing, such as at Priest
Rapids Hatchery, seems to produce adult Brights that are smaller,
younger, and more likely to be male than their natural counterparts.
The net result is that the average hatchery fish may have only about
0.80 of the reproductive potential of the average natural fish.
Changes in growth conditions in the ocean probably did not contribute
to the change in size, although the ocean fisheries of Southeast
Alaska and British Columbia appear to select, in the genetic sense,
against large size and old age in Brights.
Since 1978, in-river commercial fisheries have caught larger
Brights and a higher proportion of females than are found in the
escapement of the Priest Rapids Hatchery component of the stock, but
the fisheries impact the two sexes differently by taking the larger
males and the smaller females. The effect on the natural component
may differ because of their apparently larger average size. I found
no evidence that larger fish or more females were caught when 8-in.
minimum restrictions were in effect on gillnet mesh size relative to
periods when mesh size was not restricted. Impounding the mainstem
during the last 50+ yr may have removed obstacles to migration (e.g.,
Celilo Falls) that selected for large size in Brights, but that
hypothesis could not be tested.
The perserverance of larger and older phenotypes in the Bright
stock suggests that countervailing selection -- perhaps during
spawning, incubation, and/or early rearing -- may have resisted the
effects of a century of size- and age-selective fisheries. That
resistance, however, may reduce the productivity of the stock.
Declines in average size and age at maturity can have
undesireable consequences. Lower average size means less biomass
landed and lower commercial value. Lower average fecundity and a
diminished ability to reproduce in some environments are also
expected. Loss of size and age classes may reduce the ability of the
stock to adapt to environmental variations.
These results are relevant to several management practices. A
holistic approach to fishery management issues is necessary to avoid
erroneous conclusions based on narrow perspectives. Measuring
reproductive potential of the catch and escapement would be superior
to the conventional practice of simply counting numbers of fish.
Many aspects of artificial propagation can be improved, including
broodstock aquisition, mating regimes, and rearing practices. Stock
abundance is a major factor in determining the effect of many
management practices on the stock. In general, fisheries managers
must be mindful that they manage very complex natural systems.