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Understanding Trout Populations through Secondary Production and P/B Ratios

In a previous post, I wrote about secondary production and how it is influenced by calcium and other nutrients like nitrogen and phosophorous. In that post, I focused on water chemistry and macroinvertebrates. And, to keep that post short(ish), I skipped more than a few thoughts that I figured I would start this post with.

  • While stream productivity may be limited by nitrogen and phosphorous, we often talk about streams as being retention limited. This is because nitrogen, phosphorous, carbon, and other nutrients entering the stream quickly flow downstream, particularly in smaller, higher gradient streams - like trout streams. Streams would be more productive if they could prevent nutrients from moving downstream. Woody debris, meanders, eddies, and other sources of in-stream complexity increase retention.

LWD increases retention
Seneca Creek in West Virginia flows through a backcountry wilderness area with lots of large woody debris (LWD) that increases retention.
  • Macroinvertebrate productivity - essentially the measure of biomass of aquatic macroinvertebrates "created" over the course of a year - is how many of the nutrients entering the stream are converted to biomass. In high calcium streams, often much of this biomass is in isopods (cressbugs) and amphipods (scuds). This is how much of a stream's primary productivity is passed on to trout - either directly through herbivorous macroinvertebrates or through predatory macroinvertebrates or fishes.

Rock from a Driftless riffle
A lot of "slime" on this rock. That slime is food for macroinvertebrate grazers.
  • Fishes in streams are not only dependent upon instream primary and secondary production but terrestrial insects play a huge role in the summer. This is part of a concept we call reciprocal prey subsidies because aquatic insects are important to spiders, birds, bats, and other terrestrial animals at other times of the year. Non-aquatic insects are incredibly important at a time when food is otherwise hard to come by. It takes a lot of midges to equal the biomass of a grasshopper! Not all productivity has to occur in a stream to effect a stream.

A late fall grasshopper from a Driftless riparian zone.

Production and Fisheries Management

When it comes to topics of fisheries, production, and harvest, William Ricker, "the godfather of modern fisheries science" is the starting point. Bill Ricker is best known for the model that bears his name (Ricker Model) which serves as a foundation for the field of fisheries stock assessment. Ricker in 1946 notes how while standing stock (biomass) is often well known and understood, we know little about production. And his - and others - early work on production lead to a host of new models such as bioenergetics models and to a better understanding of fisheries. In particular, at this point in history, we are looking for a better understanding of how to feed a growing population and have the knowledge of past fisheries collapses, not that we don't repeat those mistakes.

Brook Trout sampling
Kirk lifting a heavy net full of Brook Trout into the barge to be weighed and measured.

Production, quite simply, is the biomass increase of population over the course of a year. Keep in mind that much of that production is lost to natural and angling mortality. Fish produce somewhere between tens and tens of millions of eggs a year, depending upon the species. Some percentage of those hatch and grow - that is create production. And some percentage - typically a pretty small one - grow to be catchable either by fisheries sampling gear (i.e. electrofishing, gill nets, etc.) or by anglers. Production tends to be high in young age classes (year 0) and lower in older age classes due to two key factors. First, there are simply many more young fishes than older fishes in almost any population. Second, older fishes tend to gain less production for their body weight (often described as grams/gram/day or year).

In fisheries management - particularly for commercially fished species - the response (dependent) variable is often biomass rather than population size. Fish are sold by weight, not by number. And because of the economic importance of commercial fisheries, recreational fisheries typically adopt models created for commercial fisheries. In the wildlife and fisheries world, mathematics and modeling are most prevalent in the management of commercial fisheries and in conservation biology where the goal is to prevent the extinction of species. Biomass is typically assumed to be an annual average measure of biomass, often estimated at a particular time when that species is susceptible to fisheries gear. This allows for meaningful comparisons among years and over time.

Fisheries histogram
An example of a Brook Trout population histogram - note that smaller fishes are less susceptible to electrofishing and are certainly underrepresented in the sample.

As with the (secondary) production of macroinvertebrates, increases in nutrients (N and P, in particularly) and buffering capacity (calcium, in particular) lead to greater secondary production of fishes. Fishes are another step removed in food webs from the base of the foodweb but ultimately, they are quite dependent upon primary production, though generally through consuming macroinvertebrates. However, in streams, a quite significant amount of a trout's food comes from terrestrial sources during the summer and not all of their food is macroinvertebrates. In general, "productive" streams in terms of fish production are much the same as the streams that produce high primary productivity and macroinvertebrate (secondary) production. Production of trout can be quite high as they feed relatively low on the trophic pyramid and they are relatively short-lived and quick growing among the fishes. As I had written about earlier, parental male Bluegills will not spawn until they are 7 or 8 years old. Exceedingly few stream trout reach their fourth or fifth year of life, let alone their seventh or eighth.

Production to Biomass Ratio

Fisheries biologists and researchers often use production to biomass (P/B) ratios as a measure of production and the effects of harvest (Waters 1992, Kwak and Waters 1997, Randall and Minns 2000). The idea is pretty simple, though it is not that easy to measure...but pretty easy to estimate. P/B ratio is simply the ratio of annual production (growth) to the total biomass of the population. Thus, we would expect relatively slow growing, long-lived fishes to have low ratios and quick growing but short-lived species to have high ratios. And within a species, differences or changes in P/B ratios may be informative about changes to the population, the effects of harvest, a particularly strong year class, or other factors that vary and alter production and/or biomass.

Brook Trout receiving a PIT tag
A fish receiving a PIT tag - weighing recaptured fishes is one of the best methods to directly estimate annual production. Chris Collier photo.

Many factors affect the annual P/B ratio of a species or a stock in commercial fisheries terminology. Certainly the are differences among species - slow growing Lake Sturgeon would be expected to have a low P/B ratio whereas Brook Trout in streams often rarely survive more than 3 or 4 years and would have a much higher P/B ratio. And a fish that has few individuals survive more than a single year - many of the "forage fishes" - would be expected to have P/B ratios well over 1. While not the focus of this post, primary producers, that is photosynthetic organisms, in aquatic systems often have very short generation times and thus very high P/B ratios. Similarly, secondary production of macroinvertebrates is often very high and on annual or even shorter cycles. In fishes, population dynamics are often regulated through density-dependent processes which means that the mortality (natural or fishing mortality) leads to an increase in production. In other words, removing larger fishes frees up resources for smaller fishes and allows them to exhibit faster growth rates.

In many fisheries, harvest is the main source of mortality, particular as it relates to the reduction of stock biomass. As above, the common wisdom is that harvesting larger fishes - particularly after they have been allowed to spawn for some period of years - allows to greater recruitment. In fisheries terminology, recruitment often is recognized as when a fish grows to a harvestable size. In recreational fisheries, the endpoint may be a particularly size that is desirable by anglers. Alternatively, recruitment is also used to describe the number or biomass of age-0 (young of the year) fishes that make it to age-1.

To measure the effects of harvest, Ricker (1946) defined a new term, the ecotrophic coefficient, as the ratio of the population's yield (how much biomass was harvested) to the annual production of the population. Based on life histories, sustainable harvest of different populations (and stocks), may be able to sustain quite different levels of harvest. A long-lived, slow growing species needs to have a low ecotrophic coefficient to remain sustainably harvested. Fast growing fishes that mature at young ages should be able withstand significantly more biomass to be harvested.

Stream electrofishing
Electrofishing is the most common method to estimate fish populations (and biomass and production) in streams.

As an example, the decline in Walleye in many northern Wisconsin lakes was evidenced by changes in P/B ratios (Rypel et al. 2015, 2018, Embke et al. 2019). Here fisheries managers and researchers often use the other meaning of recruitment, that is fishes that move to be an "age-1" fish. In other words, the number of fishes that survive that first year of life which has the highest mortality rates. Life is always tough for little fishes! What Rypel et al. (2018) found in studying Northern Wisconsin lakes was that natural recruitment lakes saw no trends in P/B ratios over time but stocking supplements and stocked (little/no natural reproduction) lakes showed significant declines from 1990 to 2012. The more dire finding was that the number of lakes the relied only on natural reproduction declined over this same time. As you may be familiar with, Escanaba Lake which has long been used as a research lake by the Wisconsin DNR and their partners. In an earlier Rypel et al. (2015) study on Escanaba Lake, they found that an annual exploitation rate (% of biomass harvested by anglers) of 20% resulted in approximately 100% of the annual production being removed in that year. Higher exploitation rates - the standard acceptable rate for Walleye had been 35% - would result in overfishing and a likely decline in production and biomass over time. Similarly, Embke et al. (2019) found that the percentage of production that was harvested was greatly underestimated and that changes need to be made to adapt to our changing climate and Walleye production dynamics. I use these studies as a way to show how production, biomass, and P/B ratios have been used to inform fisheries management decisions. The issues with Walleye are that there is less recruitment and those that do grow to harvestable sizes are often being overharvested.

The P/B Ratio and Trout

There are relatively few studies using P/B ratios (Layman and Rypel 2020) and fewer for stream trout (but see Waters 1992, Kwak and Waters 1997, and a few other papers). Fortunately, of these few trout studies, a significant percentage of these studies have been from our backyards (more below).

Fantail Darter
A Fantail Darter, a member of the perch family (Percidae). They tend to be short-lived which would result in a high P/B ratio.

First, to put trout P/B values into context, P/B ratios were estimated for 79 freshwater fishes in eastern Canada (Randall and Minns 2000). P/B ratios ranged from lows just over 0.1 for Atlantic Salmon, Chinook Salmon, and Muskellunge to values over 2 for a number of darters, minnows, and sticklebacks. The highest value was 4.0 for the Bluenose Shiner. Brown Trout P/B ratio was estimated at 0.34 and Brook Trout, significantly higher at 0.66. To make one more comparison, Largemouth and Smallmouth Bass P/B ratios were similar at 0.33 and 0.32, respectively, and Bluegill were estimated at 0.85.

Some of you probably knew Dr. Tom Waters, a fisheries researcher at the University of Minnesota that is probably best known for his book, Sediment in Streams. He was a pioneer in applying production, biomass, and P/B ratios in trout streams. His seminal paper in the area (Waters 1992) found quite high P/B ratios of 1.5 for Brook Trout and 1.0 for Brown Trout. He credits the high values to the productivity of the Driftless stream he studied and the differences between the species to the fact that Brown Trout generally had several additional year classes compared to the maximum of 4 year classes for Brook Trout in this study. Tom Kwak, a graduate student of Dr. Waters, followed up with his dissertation study, finding quite similar P/B estimates. These estimates were also similar to those of Hunt (1974) and papers by Brynildson and Brynildson (1984) and Brynildson and Mason (1975) in Wisconsin. Of note in these Midwestern stream, production and biomass are quite high compared to most other places.

Gratuitous fish picture
Seemed like time for a fish photo...

Where things get more interesting are adding in the effects of harvest and making global comparisons. In general, fishes with high P/B ratios would expected to withstand higher harvest compared populations or species with lower P/B ratios. And the streams we are talking about are quite productive. You probably noticed that the estimates from Midwestern streams are quite significantly larger than those from eastern Canada (Randall and Minns 2000). First, I'd assume that the productivity of Midwestern streams and the temperature regimes / growing season compared to eastern Canada are quite different. Second, there is a ton of annual variation in these numbers.

Brown Trout on a grill
Harvesting some of the Driftless trout production and biomass..

Waters (1992) tried to revive Ricker's ecotrophic coefficient (EC) and suggests it is a simple and effective way to assess the effects of harvest. Interestingly, it received little attention until these last few years when researchers like Lyman and Rypel (2020) and others have again revived this nearly 80 year old idea because it has been useful in understanding how lack of recruitment (poor year-0 survival) and overharvest have negatively affected recreational fisheries (Rypel et al. 2015, 2018). Waters uses the ecotrophic coefficient (harvest / production) as a way measure the effect of harvest on trout populations. He recognized that Brook Trout, highly susceptible to angling (aka dumb) can have EC values of 0.5 to 0.75 (remember not all mortality is from angling). And that regulations can have an affect on the EC and P/B ratios. Reducing bag limits, size limits, catch and release regulations, "one-over" slots, and other regulations can be ways to lower the EC. And in fisheries where harvest may help increase productivity, relaxed bag and size limits may take advantage of inherent productivity of streams (for more, read Fisheries Management Topics: Regulations).

I often write these blog posts because they are research topics I am interested in and this helps me get my mind around an idea. This being one of those times. We - that is the Midwest - have a wide variation in productivity among our streams. In my part of the Driftless, our productivity is "sky high" but go a little further north and productivity (I think...) is quite a bit lower. And here in the heart of the Driftless, there are a number of different regulations so we can test how P, B, and P/B ratios are affected by regulations, weather, flood events, and any number of other factors.

Literature Cited

Brynildson, O. ML, and Brynildson, C. L. (1984). Impacts of a floodwater-retarding structure on year class strength and production by wild brown trout in a Wisconsin coulee stream. Wisconsin Department of Natural Resources Technical Bulletin 146.

Brynildson, O. M. and Mason, J. W. (1975). Influence of organic pollution on the density and production of trout in a Wisconsin stream. Wisconsin Department of Natural Resources Technical Bulletin 81.

Hunt, R. L. (1974). Annual production by brook trout in Lawrence Creek during eleven successive years (No. 82). Department of Natural Resources.

Waters, T. F. (1992). Annual production, production/biomass ratio,and the ecotrophic coefficient for management of trout in streams. North American Journal of Fisheries Management, 12:1, 34-39, DOI: 10.1577/1548-8675(1992)012<0034:APPBRA>2.3.CO;2

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