Science Friday: What happens when you cram the big’uns in with the small’ins?
We sure do love this beautiful weather! It’s almost the first day of June. Summer is officially within sight.
This week’s Science Friday goes back in time over 20-years to 1997. We review a study conducted by Brett Harvey and Rodney Nakamoto. We have reviewed some of their work previously, which focused on habitat usage by adult steelhead. Today we touch on the juvenile life stage. The full study can be read here: https://www.fs.usda.gov/treesearch/pubs/7776
They conducted a six-week experiment in North Fork Caspar Creek, located on the coastline between Mendocino and Fort Bragg, California. They fenced off the stream into eighteen separate sections, which prevented fish from moving between the experimental stream sections. The sections were classified as shallow (< 3-4” average depth) or deep (8-10” average depth).
They stocked each section with two size-classes of steelhead that represented age-0 (average of approximately 2” in length) and age-1 fish (average of approximately 4” in length). Hereafter we refer to the fish by age rather than size. Steelhead were stocked in three different combinations, including 1) only age-0 fish, 2) only age-1 fish, and 3) half age-0 fish and half age-1 fish. Total biomass of juvenile steelhead was the same in all the sections, only the composition of age classes differed. After six-weeks the author’s collected all the fish in each enclosed section and evaluate the growth of fish in stream sections with differing depths and composition of juvenile steelhead.
The study revealed some interesting insights about how differences in habitat and interactions among age-classes can influence growth of juvenile steelhead.
First, growth of age-1 steelhead was greater in treatments in which age-0 steelhead comprised half the total biomass of fish than in treatments where there were only age-1 steelhead. This makes sense if we think about it. In a treatment with only larger fish, the larger fish grew more slowly, presumably because there was more competition among larger fish for available food and space. In contrast, when many of the larger fish were replaced with younger and smaller fish, they were able to grow faster because they could easily outcompete the smaller fish for food.
Second, however, the relationship changed when the fish were in shallower water. In that case the growth of age-1 steelhead in shallow habitat was lower when the smaller and younger age-0 fish were present. This also makes sense. Larger and older steelhead are likely to be less effective feeders in shallow water that is more difficult for them to navigate, especially when there are lots of smaller competitors that are not disadvantaged by shallower water.
Lastly, growth of age-0 fish was unrelated to habitat or the presence of age-1 fish. Again, this likely occurs because the age-0 fish are able to more easily navigate shallower waters and because they may feed on slightly different food sources than larger and older Mykiss.
This is a lot of information to absorb. The implications are many, but to us, it really highlights the value of having diversity in habitat conditions, especially depth. Having both shallower and deeper habitats allows juvenile O. mykiss of different sizes to partition the habitat. When larger fish are forced into shallower habitats, where they compete with smaller juveniles, they are at a competitive disadvantage. Ultimately then, restoring habitat characteristics and processes is not just about providing better habitat for all fish equally. Rather, it is about creating diversity in habitat, and ensuring that there is enough variation in habitat to support a diverse population of fish.