Scaling in Fisheries Management, Part Two: Conceptual Models

In the last post, I discussed scaling in fisheries habitat science. I finished by talking about limiting factors, or the variable that is not allowing a fish population to grow. Often, determining what is actually limiting a population's growth is incredibly difficult and can be different for fishes of different ages, species, and locations. In that case, how do fisheries biologists determine what actions to take to try to help struggling populations and grow their populations? Well, we start with conceptual models. Unlike some models that provide specific, quantitative predictions and are often used at the expense of real, on-the-ground data collection, conceptual models just provide a hypothesis. Conceptual models are basically a set of qualitative ideas about how fish populations respond to different variables. For example, a simple qualitative model might predict that land uses such as grazing, farming, and urbanization will impact water quality metrics such as sediment and temperature which will in turn affect the parameters of population growth (see Fig. 1). It does not make specific predictions about how much sediment input will be changed by grazing or how a food supply will be impacted. It simply suggests there might be an impact.

Figure 1. Conceptual Model of land uses on population growth

Once a conceptual model like this is created (and it can simply be an idea not a drawn out diagram),
biologists can decide what parameters of each section are having the greatest impact on the next section (i.e. that grazing is causing increases in temperature that is causing higher juvenile mortality). This provides a starting point for a biologist to make an alteration in a system and watch the response of the population. If the population does not respond as predicted, the biologist will need to rethink the conceptual model or determine if a different parameter is responsible. 

Conceptual models can be especially useful at large scales where climate, hydrology, and geology all impact streams in different ways and have impacts on large and small scale parameters. 

Scaling in Fisheries Management, Part One: The Limiting Factor

This is part one of a four part series on scaling in fisheries biology. 

Humans are often biased by what they can see with they're own two eyes. People are a lot more like Doubting Thomas than most would like to admit. Accepting that the Earth was a sphere was difficult for most people because we only perceive the Earth as being flat. But, in eventuality, the true nature of a problem tends to come out. In fisheries, we have traditionally focused on fish that live in a relatively small area (a reach) and applied what we learned there to an entire population. For example, biologists might snorkel a small section of a creek and observe what type of habitat cutthroat trout are using. If,  for example, they find that 90% of the trout are hiding in woody debris even though the creek habitat is only 10% woody debris, they might assume that cutthroats require woody debris. While this makes intuitive sense, it is often the case that adding woody debris to a stream doesn't end up increasing the trout population. Well, what's going on here? If the fish aren't responding to more woody debris, what is actually happening. Evidently, we haven't found the variable that is limiting the cutthroat trout population. In other words, something other than woody debris is not allowing the population to grow any larger. Whatever is actually limiting the population is called the limiting factor. Now determining what the limiting factor for a population is can actually be extremely difficult. There are tons of possible variables that might be responsible. How do we know if it is a lack of spawning gravels, or woody debris, or increasing temperatures, or sediment input, or fertilizer run-off, etc., etc.? Well...we usually don't. There is often just no possible way to determine what of a great number of interacting and interwoven variables is limiting. That is where conceptual models come in...yes, models. The term I usually use as a dirty word. More on that in the next post.

Simplicity in Fishing and Fisheries: The Tenkara Way

Tenkara. The ancient form of fly fishing developed in the mountain streams of Japan. What makes tenkara different from American/European fly fishing? Well, mainly the lack of a reel. Instead, a woven fly line is attached to the end of a long, telescoping rod at its terminal end. At the end of the fly line, a single piece of clear monofilament is attached and a fly tied on at the end. Perhaps most puzzling to western anglers is that true tenkara masters only use a single fly pattern! There's good reason for this however. In our modern "match-the-hatch" fishing craze, we often obsess about getting the fly to exactly match whatever it is that seems to be landing on the water. With tenkara, the focus moves away from exact imitation of a physical appearance to one on the action of the fly in the water.  In other words, tenkara anglers are focusing on their fishing skills and not compensating for inexperience with a three dollar piece of foam. In fact, tenkara fishing is so simple in its design, that a person can get out and fishing with nothing but a rod, a fly, monofilament, and a line. No need for overstuffed vests or bulky chest packs. I like to grab my setup, which I can carry by hand, and take the bus to the local river. It's beauty is in its simplicity.
First trout caught on a tenkara rod.

In modern ecological science, we have become obsessed with creating complex mathematical models and statistics that allow us to find significance where there is none. Now, don't get me wrong, there is a place for models and fancy statistical methods, but I believe they are overused at the expense of important naturalistic observation. I propose that we ecologists need to take a lesson from Tenkara. The most efficient and most beautiful way to do things is often the simplest way. Let me provide a more concrete example. In modern salmonid management, vast resources are spent in an effort to predict exact recruitment numbers, fecundities, and food consumption in many systems. While valuable in many cases, little is known about a great many species that interact with these salmonids. Maybe if we knew some of the basic behaviours and movement patterns of the non-game species in these systems, we would have a better understanding of how to manage the salmonids as part of an entire community animals.

It is understandable that scientists want to be on the cutting edge of their field. Doing things that could be done with the technology of fifty or even a hundred years ago does not wow other academics. But if we want to understand how to manage fisheries and not just fish, we need to understand the basic, observable characteristics of all species in a community, not just have very complex models of a single species. Maybe it's time to get back to the basics.

This post was sponsored by the Tenkara Rod Co. of Driggs, Idaho who generously provided me with a tenkara fishing outfit free of charge. If you would like to experience tenkara fishing for yourself, I highly recommend trying out the high quality products from the Tenkara Rod Co. However, all opinions in this essay are my own and do not necessarily reflect the views of the Tenkara Rod Co. or any individual other than myself.

The Iceland Study -- Global Change and Streams

National Science Foundation video about Dr. Wyatt Cross et al.'s study on climate change in Iceland streams. I was involved with this study last year helping to process invertebrate samples from the different streams.

Fisheries Management Databases

I recently read a paper as part of Dr. Andrea Litt's lab at Montana State that called for a new database where fisheries & wildlife scientists can deposit their reports on various management actions. This paper was published in 2004 and to my knowledge such a database still does not exist. While I lack the skills/time to create such a database, I've decided to compile a few links to different report collections relevant to Montana fisheries.

Montana Fish, Wildlife & Parks

Montana Department of Environmental Quality

Montana Natural Heritage Program

Fisheries Food Web Management

Food webs (interconnected food chains) are a well established concept in ecology. Traditional fisheries management has incorporated food webs for quite some time, recognizing that fish of interest have predators and can control small-fish populations. However, managing for production of small invertebrates that many stream fishes feed on is virtually nonexistent. At first, this would seem an obvious way to manage stream fish populations...change the amount of food available to the fish, change the number of fish. Unfortunately, there are some very good reasons why this isn't done.
Overall, I think stream food web management incorporating invertebrates has enormous potential as a tool to aid fisheries managers in controlling and supplementing fish populations. However, a number of questions need to be addressed before practical application can be implemented. Here are some that I've been thinking about:
Example of a river food web from Cross et al. 2011
Firstly, it can be very difficult to tell when a fish population is limited by the amount of food available to them. A good indicator is length-to-weight ratios of a population but even high numbers (indicating skinny fish) don't exclude other factors such as temperature and stress that could affect fish health. Secondly, figuring out all the different parts of a complete food web is incredibly complex. Sampling enough invertebrates in a stream section to estimate production is a ton of work by itself. But sampling invertebrates in a stream is only a part of building a food web. You also need to estimate population sizes and production of fish populations and then figure out which invertebrates the different fishes are feeding on and in what proportions. Lastly, if you finally have constructed a complete food web and have found that some invertebrate food source is limiting fish populations, you still need to decide what action to take to solve the problem. That would usually include some type of habitat management to create favorable conditions for the invertebrate of interest. Both building a food web and altering habitats can be very expensive.

  • When is a food web study necessary?
  • Can you focus on a small segment of food web to decrease time and monetary requirements and still make accurate predictions?
  • What species are socially valuable enough to get funding for a food web study?
  • Will food web management for "valuable" species adversely impact other native species?
  • How do you manage habitat for taxon-specific invertebrate production?
Here is a link to Dr. Wyatt Cross' webpage detailing one of the leading stream food web studies currently going on:

Original Manuscript: The Impacts of Urbanization on Leaf Decomposition

Here is a link to an article I wrote about a study my fellow students and I conducted last fall. The work is open-access and free to share. Note that the manuscript has not been peer-reviewed however. Feel free to comment below!

The Impacts of Urbanization on Leaf Decomposition PDF