Biological interactions potentially alter the large-scale distribution pattern of the small pelagic fish, Pacific saury Cololabis saira

Abstract

We aimed to understand the effect of biological interactions on the distribution of small pelagic fishes. Surveys were conducted during 2003-2019 using sea surface trawl nets in the western and central North Pacific Ocean, covering an area between 143° E and 165° W, to estimate the distributions of 4 small pelagic fishes: Pacific saury Cololabis saira, Japanese sardine Sardinops melanostictus, chub mackerel Scomber japonicus, and Japanese anchovy Engraulis japonicus. These species are potential competitors, as they all occupy a similar niche. Japanese anchovy was abundant during 2003-2012, while Japanese sardine and chub mackerel were abundant during 2013-2019. Only Pacific saury was distributed throughout the survey area, while the others occurred within the area west of 180° longitude (WA). Pacific saury and its potential competitors showed adjacent distributions, with a slight overlap along sea surface temperature gradients in the WA. This distribution pattern remained even when Japanese sardine expanded its distribution to colder waters and increased in abundance after 2013, resulting in a shift in the distribution of Pacific saury to colder waters and a period of high densities of Japanese sardine. Such a distribution shift of Pacific saury was not observed in the area east of 180° longitude, where no Japanese sardine was observed. These results suggest the possibility that biological interactions such as inter-species competition can be an important factor in determining the distribution of small pelagic fishes.

Fish weight reduction in response to intra‐ and interspecies competition under climate change

“…Apart from the direct competition for food, Fuji et al. (2023) suggested that the dominant species would competitively exclude other species, forcing them to shift distribution, possibly leading to an increase in energy loss because of longer migration routes.…”

“…The common trends derived from the ‘medium‐term period’, which included more species and excluded the influence of abundant Japanese sardine populations in the 1980s, were negatively correlated with the summed biomass dominated by chub mackerel and Japanese sardine. Since 2013, the Japanese sardine and chub mackerel have been increasing in both the Pacific (Fuji et al., 2023) and the Sea of Japan (Kuroda et al., 2021a; Muko et al., 2021) and have dominated other species in biomass. This observation indicates that high stock levels of Japanese sardine and chub mackerel might trigger intra‐ and interspecies competition.…”

“…In addition, it has been shown that their diet is more determined by the available planktonic prey sizes (Bachiller & Irigoien, 2013). Many studies suggest evident intra‐ and interspecies competition (Fuji et al., 2023; Kamimura et al., 2021, 2022; Kishida & Matsuda, 1993; Nishikawa et al., 2022) and high trophic niche overlap among them (Bachiller & Irigoien, 2015; Kim et al., 2023; Yasue et al., 2011, 2014). Therefore, in this study, we summed biomass of the three dominant species to quantify the intensity of competition for the limited zooplankton resources.…”

“…The reduced chlorophyll-a concentrations following the 1976/1977 climatic regime shift and the flourishing of Japanese sardine have been suggested to have caused a decrease in summer mesozooplankton biomass in the 1980s in the Oyashio waters (Ito et al, 2007;Tadokoro et al, 2005) and the Sea of Japan (Kodama et al, 2022). Apart from the direct competition for food, Fuji et al (2023) suggested that the dominant species would competitively exclude other species, forcing them to shift distribution, possibly leading to an increase in energy loss because of longer migration routes.…”

“…This observation indicates that high stock levels of Japanese sardine and chub mackerel might trigger intra-and interspecies competition. The rising biomass induced the intra-and interspecies competition for the Japanese sardine and chub mackerel (Kamimura et al, 2021(Kamimura et al, , 2022 and influenced other species, including the Japanese saury and Japanese anchovy (Fuji et al, 2023). The increasing biomass of the Japanese sardine and chub mackerel may also lead to competition with blue mackerel, Pacific cod and amberjack during their early life stages, given that all of them prey on zooplankton and their feeding grounds partially overlap (Table S1).…”