Aging fish otoliths recovered from Pacific harbor seal fecal samples

Fishery Bulletin, Oct, 2006 by Susan D. Riemer, Robert Mikus

Discussion

We have shown in the present study that 1) Dover sole otoliths recovered from harbor seal scats can be successfully aged, and 2) seals in the Alsea River consumed Dover sole that ranged in age from one to 12 years. Interestingly, the highest frequency of occurrence for Dover sole in our collections (ODFW (5)), and the greatest range in their age (Table 1), occurred during summer--a period when Dover sole move into shallow nearshore waters (Hagerman, 1952; Markle et al., 1992). This migration into nearshore waters brings fish into contact with an increasing population of pinniped predators.

The majority of Dover sole otoliths recovered from scats were those of fish younger than four years. In contrast, Dover sole landed in Oregon's commercial fishery between 1984 and 2004 were aged to be between three and 60 years old and the average age was 13 years (ODFW (5)). Of the 45,026 fish aged from the commercial fishery, over 50% were ages similar to those recovered from seal scat samples (four to 12 years) and had an average length of 33.4 cm. Of the fish checked, 79% were identified as mature. However, the majority (70.5%) of otoliths analyzed in our study were from one, two-, and three-year-old fish, whereas only one of the commercially caught fish was in this age range. Four- to six-year-old fish made up 0.3% of the commercially sampled fish and seven- to 12-year-olds made up the largest portion at 53.5%. This lack of younger age fish in the commercial fishery is partly due to the discarding of smaller unmarketable fish at sea. Sampson and Wood (2001) reported that from 1956 through 2000, between 5% and 14.6% of the commercial Dover sole catch at sea was discarded because of the small unmarketable size of the catch. What effect a consistent removal of older age fish and the discarding of younger fish by the commercial fishery, coupled with the take of younger age fish by an increasing number of predators could have on the population as a whole, is unknown.

Many species of marine fishes other than Dover sole contribute to important commercial fisheries in Oregon (ODFW (6)). Estimating prey age from fish otoliths recovered from seal and sea lion scats will be useful for researchers when describing impacts on long-lived marine species such as rockfish, which may be at greater risk to this long-term and increasing predation by coastal pinnipeds. Love et al. (2002) reported that rougheye rockfish (Sebastes aleutianus) live to at least 205 years and yelloweye rockfish (S. ruberrimus) to 118 years, whereas other species such as calico (S. dalli) and squarespot (S. hopkinsi) may only live for a decade or two. These older fish in some cases also have a late maturity; for example, only half of all yelloweye rockfish are mature at an age of 22 years.

As the sustainability of these species and the fisheries that target them become difficult to manage, there will be more interest in the impact of predators on these fish populations. Determining fish age has enabled biologists to more accurately estimate appropriate fishing levels, particularly for rockfish species (Love et al., 2002). This information can also contribute to assessing the impact of pinnipeds on these species and can improve stock assessment models. Otolith age can provide information on the size of fish consumed when the age-length composition in the population or subgroup is available (Salthaug, 2003). In addition, age estimates provide information on the reproductive maturity of the prey consumed. Aging will also provide a more accurate estimate of the MNI of a particular prey species consumed than an estimate based on the maximum number of left or right otoliths of a similar size, particularly because different age fish can have otoliths of approximately the same size.