Numerical and Economic Contributions of Wild and Hatchery Pink Salmon to Commercial Catches in Japan Estimated from Mass Otolith Markings

Hatchery programs involving mass release of artificially propagated Pacific salmon have been implemented to supplement wild populations and to increase fishery harvest around the North Pacific rim. Evaluating the contribution of wild and hatchery fish to a fishery is essential to understand economic feasibility and the impact of hatchery fish on the ecosystem (Ruggerone et al. 2010). However, precise estimates are often difficult to obtain, particularly when hatchery and wild fish are mixed in the catch.

In Japan, it has been assumed that almost all Chum Salmon Oncorhynchus keta and Pink Salmon O. gorbuscha originate from hatcheries (Hiroi 1998; Kaeriyama 1999). However, a large contribution of wild fish is suspected because the Pink Salmon catch often oscillates biennially (i.e., 2-year life cycle), wherein significant differences in abundance persist between odd- and even-numbered years, despite the constant number of fry released (Figure 1). The contribution of wild Pink Salmon to Japanese commercial catches was estimated by a population model to be approximately 80% in the most recent decade (Morita et al. 2006). However, no empirical study has quantified the contribution of hatchery and wild Pink Salmon to the fishery catch in Japan.

The objective of this study was to quantify the numerical and economic contributions of hatchery and wild Pink Salmon to the mixed-stock commercial fishery in Japan by identifying the ratio of otolith-marked hatchery fish to unmarked and thus presumably wild fish.

METHODS

We examined the contributions of hatchery and wild Pink Salmon to the commercial catch between July and November from the Hokkaido coast, a Japanese Pink Salmon management unit where almost all coastal Japanese Pink Salmon are caught (Morita et al. 2014; Figure 2). Pink Salmon are caught along the coast using stationary traps. A portion (∼2%) of the coastal catch occurs during the spring before June, but this shows a 2-year cyclic pattern different from the normal pattern, and these fish do not originate from Hokkaido rivers. The Japanese offshore drift gill-net fishery also catches Pink Salmon, but most originate from Russian rivers (Nagasawa 2011).

The brood years targeted in this study were 2009 and 2010 (Table 1). Pink Salmon fry released from Hokkaido, Sakhalin, and Iturup islands can contribute to the commercial Hokkaido fishery. A proportion of the hatchery fry from each island were released with otolith marks (Table 1). Three methods were used to mark otoliths: thermal marking, the dry method, and the alizarin complexone staining method (Akinicheva and Volobuev 2010; Takahashi et al. 2010; Akinicheva et al. 2011; Sato et al. 2011). Each island hatchery has a specific otolith-marking code.

Adult Pink Salmon were bought in situ from the commercial fishery of each fishing region in 2011 and 2012 (age at maturity of Pink Salmon is fixed at 2 years). The numbers of samples by season and region were determined a priori from the average catch for odd or even years to obtain random samples with respect to season and region (Table A.1 for 2011 and Table A.2 for 2012 in the Appendix). Thus, the number of samples by region i and season j were determined as 1,000 × s_i,j, where 1,000 is the total number of target samples and s_i,j is the proportion of fishery catches by region i and season j in normal years. Although our samples were not completely random because of annual variations in migration timing and regional variations, our adult samples in 2011 (Table A.1) and 2012 (Table A.2) were approximately representative of commercial catches.

The contribution of hatchery fish to the commercial catch was estimated as p/q, where q is the proportion of fry marked at release, and p is the proportion of adults marked upon their return. We accounted for the contribution of hatchery fish from each island to the Hokkaido fishery. Then, the remainder were considered naturally spawned wild fish. The total economic yield of Pink Salmon in each year was obtained from the Hokkaido Union of Fishery Regulations Committee (unpublished data). The economic yield used in this study was the exvessel value; that is, the price received by the fisher at the point of landing. Economic yield per release of Hokkaido hatchery Pink Salmon was calculated for an analysis of economic feasibility as: (total economic yield × estimated proportion of Hokkaido-origin hatchery fish)/(number of fry released from Hokkaido hatchery).

RESULTS

Of the 978 and 995 Pink Salmon examined in 2011 and 2012, 32 and 71 individuals, respectively, were otolith-marked hatchery fish (Table 2). All otolith-marked fish in 2011 were from Hokkaido hatcheries but some included Sakhalin and Iturup hatchery fish in 2012 (Table 2). In total, the contribution of hatchery fish to the total catch in the Hokkaido fishery was estimated to be 16.6% and 26.4% in 2011 and 2012, respectively (Table 2). Thus, the majority of the commercial catch originated from naturally spawned wild fish. Economic yield per release by Hokkaido hatcheries was 2.2 yen (¥2.2) (≈US$0.022) and ¥1.5 in 2011 and 2012 (Table 2).

DISCUSSION

We empirically demonstrated that naturally spawned wild Pink Salmon are the main contributors to the commercial fishery in Japan. Our quantitative estimates were generally in support of the theoretical modeling of Morita et al. (2006). Because it has been assumed that most Pink Salmon in Japan originate from hatcheries (Hiroi 1998; Kaeriyama 1999), no fisheries management measures have been taken to account for escapement or natural reproduction in Japan (Morita 2014). Although some studies have reported the substantial occurrence of wild Pink Salmon upstream from the hatchery weir (Torao et al. 2010, 2011; Iida et al. 2014), no quantitative evaluation of escapement has been conducted. A measure of escapement and conservation of natural spawning must be developed as soon as possible for the sustainable use of Pink Salmon and to prevent overfishing.

Despite the constant number of fry released, the Pink Salmon catch fluctuates greatly. These fluctuations can be largely explained by year-to-year variations in natural spawning because population trends were linked to rainfall in the fall of the two previous years (Morita et al. 2006; Figure 3). High rainfall increases river discharge, which would increase escapement prevented from the hatchery weir that is usually deployed downstream from the spawning area (Morita 2014). In addition, high rainfall increases the available spawning area and increases dissolved oxygen concentrations (Wickett 1958). Studies that examined the abundance of natural Chum Salmon reproduction showed that high water levels increased the number of carcasses of naturally spawned Chum Salmon upstream from the weir (Ito et al. 2005), as well as the number of returning wild salmon in the next generation (Morita et al. 2013).

The economic yield per release of Pink Salmon was estimated to be ∼¥1.5–2.2. The economic cost per Chum Salmon released is ∼¥5 (Kitada 1999) and the economic cost per Masu Salmon O. masou fry released is ∼¥4.6 (Miyakoshi et al. 2004), but the economic cost per Pink Salmon fry released has not been reported. The cost per release would be lower for Pink Salmon than for Chum and Masu Salmon because of the shorter feeding period in the hatchery. Nevertheless, the economic yield per release would unlikely outweigh the cost per release in the Japanese Pink Salmon hatchery program. Moreover, the net benefit should be evaluated after accounting for the negative impact of hatchery-released fish on wild fish production (Hilborn 1998). Hilborn (1998) reviewed the economic costs and benefits of nine marine stocking programs. In his review, costs outweighed economic benefits in most cases, except for the Japanese Chum Salmon program. In contrast, Kitada (1999) argued that economic benefits outweighed costs in most Japanese stocking programs, although his analysis did not address the negative impact on wild fish production and assumed that 100% of Chum Salmon originated from hatcheries.

Although there is no doubt that wild Pink Salmon significantly contribute to the commercial fishery in Japan, our quantitative estimates include uncertainty because the proportion of fry marked upon release is not 100%. At present, implementing thermally induced otolith marking is limited to national hatcheries. If the fry-to-adult survival rate was higher (or lower) for private hatcheries than that for national hatcheries, the contribution of wild (or hatchery) fish would be overestimated. In addition, some Hokkaido hatchery-origin Pink Salmon may be contributing to Russian fisheries. Further investigations using different methodologies, such as natural otolith microstructure and stable otolith isotope ratios (Barnett-Johnson et al. 2007; Tomida et al. 2014) and a broader sampling area, are required to more precisely quantify the contribution of wild and hatchery salmon to the mixed-stock fishery.