Reproductive success of jack and full-size males in a wild coho salmon population

Abstract

Despite the wealth of research on Pacific salmon Oncorhynchus spp. life histories there is limited understanding of the lifetime reproductive success of males that spend less time at sea and mature at a smaller size (jacks) than full-size males. Over half of returning male spawners can be jacks in some populations, so it is crucial to understand their contribution to population productivity. We quantified adult-to-adult reproductive success (RS) of jacks and their relative reproductive success (RRS) compared to full-size males in a wild population of coho salmon in the Auke Creek watershed, Juneau, Alaska. We used genetic data from nearly all individuals (approx. 8000) returning to spawn over a decade (2009-2019) to conduct parentage analysis and calculate individual RS. The average adult-to-adult RS of jacks (mean = 0.7 and s.e. = 0.1) was less than that of full-size males (mean = 1.1 and s.e. = 0.1). Jack RRS was consistently below 1.0 but ranged widely (0.23 to 0.96). Despite their lower average success, jacks contributed substantially to the population by siring 23% of the total returning adult offspring (1033 of 4456) produced between 2009 and 2015. Our results imply that jacks can affect evolutionary and population dynamics, and are relevant to the conservation and management of Pacific salmon.

Keywords: life history; mating systems; reproductive success.

Figure 1.

Map of Auke Lake system in southeastern Alaska near Juneau. Auke Creek weir is located between Auke Lake and Auke Bay.

Figure 2.

Number of returning full-size (male and female) and jack coho salmon counted at the Auke Creek weir from 1980 to 2018.

Figure 3.

Auke Creek coho salmon life-history diagram. Circles indicate return years (2009–2019). Individuals in this population return 2–4 years after being spawned. Thick black lines relate a single brood (parent) year to all possible offspring return years. A thin grey line relates a single return year to all possible brood years. The number next to each circle indicate the age of the fish. The number before the decimal place is the years spent in freshwater and the number after the decimal point is the years spent in the ocean. For example: 1.0 indicates a fish that spent one year in freshwater and less than a year in the ocean.

Figure 4.

Number of Auke Creek coho salmon jacks and full-size males and their offspring from 2009 to 2015. The two panes on the left (bars with black fill) include individuals that did not produce offspring while the two figures on the right (bars with white fill) do not.