migration

Culvert

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Three perched culverts (photo credit: USFWS).

A culvert is essentially a tunnel to pass flowing water, typically a small stream, under a man-made structure, usually a road.  Depending on the size, placement, and design of a culvert, the impacts to a stream channel vary.  These impacts, in turn, can affect fish habitat and fish migration.

Culverts are frequently barriers to fish movement.  If culverts are perched, where the outlet is higher than the elevation of the downstream water, fish are required to leap up into the culvert (if they are able) to continue moving upstream.  Often, culverts are not installed to be perched from the outset, but the situation is caused by erosion at the outlet of the culvert from high flows which scour the channel bed.  This type of culvert creates a downstream pool, changes the flow velocity and habitat type, and, consequently, can alter the fish community as well.

Local, state, and federal managers often work to replace or retrofit culverts that have significant ramifications for fish passage.  The U.S. Fish & Wildlife Service’s National Fish Passage Program is one example.

Phenology

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Phenological events triggers Sockeye Salmon migrations (photo credit: USFWS).

Phenology is the study of seasonal or periodic cycles in ecosystems.  It is, essentially, “nature’s calendar.” For fish, phenology is often linked to timing of important life events such as spawning, migrations, and hatching.  These events are triggered by non-biological factors like day length, temperature, and precipitation.  If the timing of the main “cue” shifts, the alteration can have significant implications for the fish populations that have evolved to optimize the timing of their life events with that factor.

Many fall–spawning Pacific Salmon populations (Oncorhynchus spp.), for example, are beginning their freshwater migrations earlier than in the past (e.g. Kovach et al. 2015).  This consistent trend across species and populations strongly suggests that a shared environmental driver (i.e., climate change) is responsible.  Unfortunately, these altered behaviors can be maladaptive – Cooke et al. (2004) found that increased pre-spawn mortality rates – sometimes in excess of 90% – for several stocks of Fraser River sockeye salmon (Oncorhynchus nerka).

 

References

Cooke, S.J., Hinch, S.G., Farrell, A.P., F., L.M., Jones, S.R.M., Macdonald, J.S., Patterson, D.A., Healey, M.C., Van Der Kraak, G., 2004. Abnormal Migration Timing and High en route Mortality of Sockeye Salmon in the Fraser River, British Columbia. Fisheries 29, 22–33. doi:10.1577/1548-8446(2004)29

Kovach, R.P., Ellison, S.C., Pyare, S., Tallmon, D.A., 2015. Temporal patterns in adult salmon migration timing across southeast Alaska. Glob. Chang. Biol. 21, 1821–1833. doi:10.1111/gcb.12829