Fresh from the press: Herring reproductive investment

Marion Claireaux, Thassya C dos Santos Schmidt, Esben Moland Olsen, Aril Slotte, Øystein Varpe, Mikko Heino, Katja Enberg. 2020. Eight decades of adaptive changes in herring reproductive investment: the joint effect of environment and exploitation, ICES Journal of Marine Science, published online, OPEN ACCESS

Abstract: Reproductive investment is a central trait for population dynamics and productivity. Fishing and environmental variations are major drivers affecting population structure, dynamics, and adaptation of life-history and behavioural traits. However, those factors are often considered independently, and few studies take into account their joint effect. In this study, we investigate the contribution of environment, fishing pressure, and intra-specific competition to variation in the reproductive investment of the Norwegian spring-spawning herring (Clupea harengus), a stock that has been fished for centuries, and monitored for decades. Reproductive investment and post-spawning weight were affected differently by growth rate (measured as mean age-at-length), sea surface temperature, North Atlantic Oscillation, and spawning stock biomass in periods with no fishing, unselective fishing, and low but size-selective fishing. We hypothesize that those changes can be explained by direct effects of exploitation such as age truncation and changes in migration patterns. Our results highlight how fishing, by affecting population-level dynamics, can modify the impact of environmental variations on life-history traits.

New paper out in ICES Journal of Marine Science! The work was part of the PhD-project of Marion Claireaux, and uses long-term data from 126 664 individual fish! Because the gonad weight (that is, the weight of eggs or milt) has only been measures since 1990, we had to reply on an indirect estimate based on the pre- and post-spawning weight of individuals of a given size. The results show that the average reproductive investment has indeed varied quite a bit within the last 8 decades:

Time series of the mean annual gonad weight (plain line) and of our proxy for reproductive investment (dots) for length classes 31–36 cm. The shaded areas represent different time periods: before the collapse where fishing was unselective (white, 1935–1969), during the collapse where fishing was anecdotal and regulated with minimum landing size (light grey, 1970–1989), and after the collapse where fishing was regulated with minimum landing size and a harvest control rule (dark grey, 1990–2015).


Moreover, our statistical analysis shows that herring reproductive investment also depends on age-at-length, which inversely related to growth rate (if an individual is relatively young in a given age, it has grown faster than an average individual, and vice versa),  population density and environmental variables such as sea surface temperature and North Atlantic Oscillation. Interestingly, the effects differed depending on the population state (pre-collapse, collapse, post-collapse):

Effect of mean age-at-length (upper row) and SSB (middle row, note that the x-axes use different scales) on the reproductive investment of NSS herring for each length class and period. For the sake of visualization and interpretation, we used the mean yearly growth (size divided by mean age-at-length) to represent the trade-off between growth and gonads rather than mean age-at-length directly. The bottom row shows how the SSB varied since 1930. Until 1970, there were no minimum size limits, and both adult and juvenile herring were caught. The fishing pressure intensified in the early-1960s and the stock collapsed a few years later. The stock started to recover with the exceptionally abundant 1983 length class and was considered fully recovered by 1989. Since its recovery, the stock has been closely monitored and managed with harvest control rule and minimum size limit has been in effect.


Currently we are working on finalizing our manuscript on herring growth within this same time period and data, so stay tuned for more!

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