Science Friday: Space, time, and maximizing habitat capacity

In Science Friday by Nick Chambers2 Comments

All steelhead, all the time, here at Wild Steelheaders United.

Over the past few weeks we have examined the scientific concepts and tools used to evaluate how productive a given stream can be for wild steelhead. Such assessments are an important component of managing a steelhead fishery — especially if the run in that watershed is ESA-listed (in the Lower 48, most are).

 

We looked at the concept of carrying capacity, how it is estimated using habitat factors, and at research indicating that carrying capacity is about more than just habitat; it is also about the fish and how they are distributed across a watershed in space and time.

 

This week we will highlight ways in which these studies have been or could be applied to steelhead management.

 

There is no dispute that high quality habitat — and connectivity among habitats — is the foundation of healthy steelhead populations. However, there is a limit to the amount of steelhead any given habitat can support before they reach density dependence, at which point fish start to decline in growth and survival.

When a watershed has reached its capacity in this regard, boosting a fish population largely depends on improving habitat conditions and/or expanding the amount of accessible habitat. Consequently, there has been much investment in restoring habitat and, more recently, dam removal. A key component of those projects is estimating, based on habitat conditions, how many fish newly restored or re-opened habitat can support.

 

The Elwha River on Washington’s Olympic Peninsula provides a good real-world example. In a landmark effort, two dams were removed, restoring access for native salmon, char and steelhead to over 70 miles of spawning habitat. Because the dams had been in place for nearly 100 years there was no way to pinpoint how many fish the river historically supported — scientists and resource managers had to estimate how many steelhead the river could potentially support now. Wild winter steelhead runs in the Elwha numbered between 150-250 fish prior to dam removal, and the restoration plan estimated the river would support approximately 6,000 winter runs within 25 years of the dams being removed. That estimate was largely based on the “intrinsic potential” of the habitat in the Elwha.

 

While habitat is crucial for healthy populations of salmon and steelhead, just because we see density dependence does not mean the habitat is being fully used to its capacity. We reviewed three studies that highlighted this issue, one on steelhead and two on Chinook salmon. Each found strong density dependence despite declines in population sizes and very little to no change in freshwater habitat conditions. In all studies the spatial distribution of spawners contracted as population sizes decreased, which is important because a patchy spawning distribution can result in high levels of density dependence early in life when small fish have a difficult time moving very far from the redd.

 

The implication is that if we don’t account for the distribution of spawners we may underestimate how many fish the habitat can actually support, because we may see density dependence even when populations are small and more habitat is available.

 

Untangling these issues is not easy, but it can be addressed through a combination of experiments, such as increasing escapement of spawners, statistical analyses, and evaluating the effects of distribution on density dependence and habitat capacity.  To help with the issue Wild Steelheaders United is working in collaboration with agency and other scientists to address the effect of spatial distribution in a series of models that will evaluate the steelhead capacity of several watersheds.

 

We also reviewed a paper on pink salmon that illustrated how slight differences in spawn timing can allow a population of fish to pack more spawners into limited habitat. In this analysis, differences in spawn timing led to differences in emergence of fry. Earlier emerging fry vacated the areas around the redd, leaving behind a void for the later emerging fry to use.

 

 

Steelhead potentially 5-8 month time period depending on the population — meaning some fish can spawn in the same habitats, just months apart, because they stagger the use of habitats for their juveniles.

 

There is very little research on this topic in steelhead. We know, however, that in places like western Washington the early-timed component of winter steelhead has been greatly depleted. In this region traditional winter steelhead fisheries started in November and ended in March. It is now rare to catch a wild steelhead in November or December, even in January in some watersheds. If the pink salmon work applies to steelhead, then rebuilding the early component of the run could effectively increase the capacity of the watershed due to the differences in spawn timing.

 

Sometimes it’s not just about time or space, but it is about both working in concert. For example, spring and fall Chinook enter and spawn in different times and places. This allows the river to support more Chinook than it would if all the fish entered and spawned simultaneously. Use of habitat is spatially different and juvenile competition is staggered over time.  Steelhead do the same with summer and winter runs, and there is some evidence that the same type of structure exists even within a population of winter runs.

 

However, it seems likely that the effects of space are greater in larger rivers where there is more habitat overall, and therefore more room for adults and juveniles. On the other hand, the effects of time may be more important in smaller watersheds where there is less room.

 

Regardless of the specific effects, the most current science suggests that estimates of carrying capacity are not always straightforward because fish have evolved ways to partition use of a watershed in space and time. Each of those sub-components of a run is important to the overall productivity, and thus, sustaining steelhead populations – and the fisheries they provide – will depend on ensuring that the populations are as diverse and productive as possible.

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