A closer look at sea lamprey mating systems.

Sea lampreys (Petromyzon marinus L. 1758) are parasitic organisms in marine water, but they use freshwater for reproduction. They dig easy detectable nests on spawning grounds. Nest counts can provide a relative estimate of population abundance (Kynard and Horgan, 2019) and, based on the average number of individuals per nest between 2 and 2.5 (Applegate, 1950; Manion and Hanson, 1980), some authors proposed to multiply the number of nests on a river by this factor to estimate the number of spawners (Gracia et al., 2016; Migradour, 2010).

Sea lampreys during mating in the River Nive (copyright INRAE_GLISE).

But sea lamprey mating systems are in fact poorly documented: they are mostly considered as monogamous throughout the literature, and little is known about sexual differences in morphology, behaviour, and about sexual selection. With that in mind, we aimed at investigating the relationship between individuals and nesting activity by combining mark–recapture and nest survey of a sea lamprey spawning ground throughout a breeding season. How many lampreys are actually using a given nest? Do each of them visit one or several nests? Does behaviour vary between females and males? And what is the real population size on a spawning ground?

The site chosen for the study is the 1 km long bypass reach of the Halsou hydroelectric power plant on the River Nive, monitored during the sea lamprey spawning period, from 6 May to 24 June 2019. Each newly captured individual was marked using two T-bar tags allowing individual recognition from resight without actual recapture.

During the studied period, we observed 202 nests built. Average number of mates per nest was about 2.3 (close to expectation from literature), with no effect of sex. The classical estimate for population size using such numbers would therefore have been about 497 individuals.

Number of nests visited per individual as a function of sex. A null number of nests corresponds to individuals only found outside of nests.

Using individual tagging on 56 males and 58 females, we were however able to go beyond these average estimates per nest. For instance, each individual visited on average more than one nest (1.65 for females, 2.26 for males). And in fact, a lot of variation between individuals could be observed. Some individuals would only visit one nest, while some other could be seen on more than 5 nests. This clearly indicates that the sea lamprey mating system may be more polygynandrous than the monogamous/polygynous system suggested from a mere observation of individuals per nest. We also found that the higher the number of nest visited, the higher the number of mates encountered. This hints at a potential role of visiting behaviour on sexual selection.

Number of nests visited and number of mates encountered for (a) male and (b) female sea lamprey. The size of the point relates to the number of individuals.

Finally, using a dynamic multistate occupancy model with augmented population (Kery and Schaub, 2011), we found that the total population size on the considered spawning ground was 177 [154; 219], a result way below the 497 individuals expected using the classic management estimation methods.  More results and data can be found in the paper by Marius Dhamelincourt and colleagues.

These first results open intriguing options: why do females visit several nests ? How much each individual, male or female, invest in nest building? Are they cooperating? And how strong is sexual selection in this species ?

References

Applegate, V.C., 1950. Natural history of the sea lamprey, Petromyzon marinus, in Michigan (Federal Government Series No. 55), Special Scientific Report – Fisheries. U.S. Fish and Wildlife Service, Ann Arbor, Michigan: University of Michigan Library.

ECP, 2018. Ecology and Fish Population Biology Facility. https://doi.org/10.15454/1.5572402068944548E12

Gracia, S., Caut, I., Carry, L., 2016. Suivi de la lamproie marine sur la Dordogne et la Garonne. MIGADO.

Kery, M., Schaub, M., 2011. Bayesian Population Analysis using WinBUGS: A Hierarchical Perspective. Academic Press.

Kynard, B., Horgan, M., 2019. Long-term studies on restoration of Connecticut River anadromous sea lamprey, Petromyzon marinus Linnaeus 1758: Trend in annual adult runs, abundance cycle, and nesting. Journal of Applied Ichthyology 35, 1154–1163. https://doi.org/10.1111/jai.13967

Manion, P.J., Hanson, L.H., 1980. Spawning Behavior and Fecundity of Lampreys from the Upper Three Great Lakes. Can. J. Fish. Aquat. Sci. 37, 1635–1640. https://doi.org/10.1139/f80-211

Migradour, 2010. Suivi de la reproduction de la Lamproie marine sur le bassin de l’Adour – Tranche 1/3, gaves et nives.

Royle, J. andrew, Dorazio, R., 2012. Parameter-expanded data augmentation for Bayesian analysis of capture-recapture models. Journal of Ornithology 152, 521–537. https://doi.org/10.1007/s10336-010-0619-4

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