The population dynamic consequence of individuals producing sufficient progeny over their lifetime for replacement is sustainability. Exploited marine populations experience elevated mortality rates due to fishing, with the result being a truncation of age structure, size,structure and lifetime reproduction. This loss of old, large individuals causes a decrease in Lifetime Egg Production (LEP), the number of eggs produced by an individual over a lifetime, and a reduction in the ability of an individual to replace itself. Estimation and maintenance of a threshold level of LEP is thus a central focus of fisheries management.

The method developed here for estimating the fractional change in LEP (FLEP) from two length frequency distributions, one early in the exploitation history and one recent, allows for inference about the changes in LEP owing to a fishery with a minimum amount of data. The FLEP estimation method was found to be unbiased at low sample sizes, but could be predictably biased in cases where assumptions of the model were violated. The method performed better than conventional means of estimating changes in LEP as it was nearly insensitive to the highly uncertain natural mortality rate parameter.

Application of the FLEP estimation method to 23 years of length frequency data for 5 species of rockfishes (Sebastes spp.) in California and Oregon suggested that LEP has been reduced to levels of concern for 4 of the 5 species. In addition, large differences in LEP were estimated over space, implying that fishing mortality is not evenly distributed.

Finally, the efficacy of managing fisheries by estimating LEP was evaluated in a modeling study for populations where maternal age dependence on larval mortality exists. In general, the effect of high mortality rates for larvae produced from young females results in small assessment errors if the contrast in larval mortality rate exists only for the youngest spawners, which contribute a small proportion of the total egg and larval production. Evaluation of a specific example for black rockfish (Sebastes melanops) suggests that maternal age effects would produce relatively small assessment errors if LEP were used for assessment.

Central Valley spring-run Chinook Salmon (CVSC) are designated threatened by state and federal authorities. Although CVSC are caught in ocean fisheries, their harvest is not actively managed, because it is assumed that measures currently in place to protect endangered Sacramento River winter-run Chinook Salmon (SWRC) will also sufficiently protect CVSC. Recoveries of tags and genetically-identified CVSC suggest these fish have a more northerly distribution than SRWC. Further, escapement data and cohort reconstructions suggest that CVSC mature later than SRWC. Thus, regulations (time/area restrictions and minimum size limits) crafted to protect SRWC alone may not adequately protect CVSC; on the other hand, regulations to constrain impacts on Klamath River and California coastal Chinook Salmon populations may also reduce impacts on CVSC. Trends in CVSC escapement were deemed acceptable in recent status updates, but concerns remain because of the negative effects caused by recent drought and ocean conditions. Should more active management of CVC be desired, current options are limited. The most promising approach is based on estimating age-specific ocean fishing mortality rates by using cohort reconstructions applied to tagged Chinook Salmon that originate from the Feather River Hatchery. At a minimum, ocean fishing mortality rates could be monitored and compared to proxy thresholds. If reference harvest rates were established, harvest models could be developed to predict how CVSC would be affected by fishing regulations, similar to the way fall-run Chinook Salmon fisheries are evaluated. Abundance forecasts would require improved juvenile production data (e.g., from genetic sampling of juvenile emigrants), since sibling-based forecasts commonly used for fall-run Chinook Salmon would not be available in time for pre-season planning. It is unclear if ocean fishing mortality rate estimates derived from hatchery proxies for natural-origin fish are truly representative, but existing data do not demonstrate obvious differences in ocean distribution or size-at-age fish. Substantial new investments in tagging or sampling would be needed to directly estimate ocean fishing mortality rates for natural-origin CVSC. Establishing specific harvest targets or limits for CVSC requires an improved understanding of production throughout their life cycle through juvenile production estimates and long-term information on spawner age structure.

https://doi.org/10.15447/sfews.2018v16iss4art4

Sacramento River winter-run Chinook salmon are listed under the Endangered Species Act as Endangered and there are substantial efforts to estimate, predict, and limit mortalities at various stages of their life cycle. One such effort is the annual forecast of the number of juvenile winter-run entering the Sacramento-San Joaquin Delta. The natural-origin Juvenile Production Estimate (JPE) is defined as the number of winter-run juveniles produced from natural spawning areas that enter the Delta, and its forecast is used to determine the allowable level of winter-run incidental take at the state and federal pumping facilities located in the south Delta. Current monitoring programs in the Sacramento Basin do not allow for direct estimation of the JPE and thus various methods have been used to forecast this value annually. Here we describe three alternative methods for forecasting the natural-origin JPE. The methods range from the status quo approach (Method 1), which expresses the JPE forecast only as a point estimate, to two other methods that account for forecast uncertainty to various degrees. A comparison of JPE forecasts for 2018 across the three methods indicates that relative to Method 1, Methods 2 and 3 result in lower JPE forecasts, by 24 and 18 percent, respectively, primarily owing to lower forecasts of the fry-to-smolt transition and the smolt survival rate occurring downstream of Red Bluff Diversion Dam. Because post-hoc estimates of juvenile winter-run abundance at the entrance to the Delta do not currently exist, we are unable to evaluate forecast skill among the three methods.