Modeling the Size Spectrum for Macroinvertebrates and Fishes in Stream Ecosystems

The size spectrum predicts abundance as a function of individual size. Size spectra research suggests strong similarity in the abundances of relatively small and large organisms from diverse ecosystems. Unlike traditional size abundance scaling relationships that use species level average sizes, size spectrum models are ataxic and can account for the size of every individual within the sample.

Demonstrating the procedure will be Brandon Gravett, Sarah Headley, and Giancarlo Racanelli, field technicians. After identifying the upstream and downstream ends of the study reach, mark the ends with removable flagging tape. Measure the width of the wetted stream channel at five to 10 transects distributed evenly along the length of the study reach, and estimate the total surface area of the study reach as the average wetted channel width multiplied by the total length of the reach.

Make a loose bowline knot in each end of a piece of polypropylene rope and wrap the rope around a tree, root, large rock, or other solid object at the up and downstream ends of the study reach as an anchor. Feed one loop through the other to create an anchor point and shorten or lengthen the rope anchor by adding or removing wraps around the anchor object as necessary. Establish a second anchor point on the opposite side of the stream as just demonstrated, and use a bowline knot to create a loop in the lines at each of the four corners of a medium to coarse mesh block net.

Using Cam Action tie-down straps, connect both sides of the top line in the block net to the anchor points, and insert the hooks at either end of the tie-down strap into the loops at the corners of the block net and the anchor points. Pull the free tether of the tie-down strap through the Cam buckle to tighten each point of contact, and pin the bottom line of the block net to the stream bank with tent stakes. Place large rocks on the side of the net facing upstream to pin the block net down to establish a seal with the bottom of the stream, taking care that the top of the net remains above water level.

Then set a second block net at the downstream end of the study reach in the same manner. To perform a fish sampling depletion pass, beginning at the downstream end of the enclosed study reach, turn on the backpack electrofisher and move through the stream in the upstream direction. Progress slowly, moving side to side throughout the study reach to ensure that all of the in-stream habitats are sampled, and have supporting crew members follow to collect stunned fish with dip nets as they are spotted.

Transfer the fish into temporary buckets, then to aerated holding tubs. Use small battery-powered bait bucket pumps with aeration stones to ensure that the captured fishes remain healthy. Pay particular attention to various small, young-of-year fishes as they are difficult to spot and capture.

The first depletion pass is complete when the upstream net is reached. For processing of the first depletion pass fish, use small dip nets to retrieve the sampled fishes from the holding tank individually or in small batches for identification, and place the specimens in white trays. Using forceps and a magnifying glass, identify each fish, then measure its total length from the tip of the snout to the end of the caudal fin on a measuring board, and weigh it on a field balance with a 0.1 or 0.01-gram precision.

Then record the species’identity and total length and weight for each specimen on waterproof data sheets. Once processed, return the fishes to a separate aerated recovery bin before releasing all of the fish downstream of the downstream block net. Select benthic macroinvertebrate sample sites within the boundaries of the fish sampling reach that are representative of the major types of physical habitats observed in the study reach.

Place an appropriate fixed area sampling device firmly against the stream bottom with the sample collection net oriented downstream and move large cobbles as necessary to establish a firm seal with the substrate. Use a brush to vigorously scrub the substrate within the sampling area for two minutes, allowing dislodged benthic macroinvertebrates to drift into the sample net. Transfer the sample contents from the net to a plastic jar and preserve the specimens in 70%isopropyl alcohol.

Then label the jar and store it in a safe location for later transfer to the lab. When all benthic macroinvertebrate and fish data have been properly formatted and plotted, a clear negative relationship between individual body mass and normalized density is often apparent. This size spectrum reflects a predictable transition from the smallest and most abundant invertebrates such as midges and small mayflies to larger caddisflies and stoneflies to fishes as shown here for samples from Slaunch Fork, West Virginia.

Similar size spectra relationships were detected for benthic macroinvertebrates in fishes in Camp Creek and Cabin Creek, two other West Virginia streams, and linear regression was used to model the relationship. The size spectra slopes were all between 1.7 and 1.8 with overlapping 95%confidence intervals. This similarity indicates that the abundance decreases as the body size increases at approximately equal rates in all three streams.

However, the differing size spectra intercepts reveal that differences in the overall density are variable among streams, but the highest densities observed in Camp Creek and much lower densities measured in Cabin Creek. As the number of size spectra studies grows, critical tests of the effects of different environmental influences on the size spectrum will become possible. It is important to remember that all fieldwork entails some risk and that electrofishing, in particular, can be dangerous.

It is therefore essential that all crew members are properly trained.