Author Archives: Katja Enberg

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!

2019 Ocean Sustainability Bergen Conference

Ocean Sustainability Bergen organised its inaugural Ocean Sustainability Conference Bergen during 21-22 October.  SDG Bergen/Sverre Ole Drønen has written a report about the conference, which you can read here. SFG was present, and Katja Enberg was invited to give a talk about her SDG214 course: “Experiences from the first SDG14 course at the University of Bergen”.

Bergen, 21. oktober 2019: Ocean Sustainability Bergen Conference 2019. Foto: Eivind Senneset, UiB

Sustainable Fisheries Group in the ESEB 2019 meeting in Turku, Finland

During this week SFG has attended the conference of European Society for Evolutionary Biology, this year organized for the first time in Finland. ESEB 2019 covered a wide variety of topics from genomics, archaic DNA, game theory, to human-induced evolution. In the human-induced evolution session Katja Enberg presented results from the PhD-project of Ingrid Wathne – you can read more about it here. Her presentation is available below:

Did you notice that annoying noise in the video? You can blame by earring. So a #ProTip: Remove hanging earrings if you will be using the headset-type of microphone… I will next time.

The meeting was very well organised, it ran on schedule, the food was good, healthy, and there was enough of it. Cudos to the organizers!

An extra bonus was the conference dinner in the Muumimaailma (Moomin world), which was a great success. Even the moomin characters where there to celebrate with us!

Henrik H. Jessen and Katja Enberg at ESEB 2019 conference hanging out with the locals.


Fishing for answers – the genetic basis for adaptation to an anthropogenic driver

In 2002 David Conover and Stephan Munch published the results from their seminal laboratory experiment on fisheries-induces evolution. Now, in the current issue of Science, Therkildsen et al. present what they found out when the pulled these fish from Conover & Munch’s experiment from the freezer and looked at the genetic details of the observed rapid changes in growth. Read here our and Theoretical Ecology Group‘s Perspective on Therkildsen’s work!

A spoiler: We think it rocks!

Conover & Munch 2002

In the original experiment, Conover & Munch (2002) found a twofold difference in body weight after just four generations of selectively harvesting either small or large individuals. The fish on the left show the size of the original population, while the fish on the top show the average size in the population where small individuals were been removed by the experimental fishing for four generations. The lowest fish show the outcome of selectively removing large fish for four generations. Photo from Therkildsen et al. 2019.