Shad, those endangered travelersby Alicia Dragotta and Claire Valleteau
Published by the April 6, 2020 on 1:52 PM
Photograph by MRM association
Shad are migratory fish that travel great distances between sea and river in order to reproduce. These long journeys are the source of great energy expenditure, particularly to find the most favourable spawn environment. These species are considered bio-indicators of our waterways. Their presence or absence indicates the ecological state of the water. Migratory distance was governed by energetics, behaviors, maturation, and upstream delays at dams. Individual adult migrant American shad (Alosa sapidissima) ascend the Connecticut River and spawn, and survivors return to the marine environment. Theodore Castro-Santos and Benjamin H. Letcher presented a simulation model of these behaviors.
The purpose of this model is to evaluate the effects of biological and physical variables on adult spawning success and survival. Only energy devoted to migration has been taken into account in the model. Physiology and energetics strongly affected distribution of spawning efforts and survival into the marine environment. Delays to both upstream and downstream movements had dramatic effects on spawning success. Other factors influencing migratory distance included entry date, body length, and initial energy content. Furthermore, dams alter reproductive success and have an impact on migration (delay).
This model suggests shad that spend more time in the river have greater spawning success but are more likely to die of energy depletion. Many important factors in the models presented here remain enigmatic. Perhaps the most important question is what causes shad to reverse direction and migrate downstream. Do both energetics and maturation play a role ?
Answering this question could be difficult but may be possible using, say, a combination of physiological telemetry (e.g., Hinch et al. 1996) and data on reproductive status, especially of downstream migrants. The purpose of this paper was to develop a management tool to evaluate the relative importance of biological and physical factors on shad reproduction and survival. Restoring access to spawning habitat by providing fish passage has been a central management strategy. Ecological continuum is very important to preserve species, including these migratory fish. Dams for example, were built for many reasons, at the origins in order to mill operations, and today for hydraulic energy exploitation. We have to reconsider the interest of these dams, remove those which are useless and adapt the others. This process has been under way for several years, opening the door to restoring access to the rivers.
Read the full study: Castro-Santos, T. and Letcher, B.H. (2010) Modeling migratory energetics of Connecticut River American shad (Alosa sapidissima): implications for the conservation of an iteroparous anadromous fish. Canadian Journal of Fisheries and Aquatic Sciences. 67(5): 806-830. https://doi.org/10.1139/F10-026
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