Skagit Bull Trout Primer Ch 3: The Repeats

Over the long pre-spawn holding period and the rigors of the actual spawning process, the adult bull trout may lose 1/3 or more of their pre-migration weight so it is hardly surprising that post spawn these fish evacuate the spawning grounds quickly. Fortunately for the Skagit fish, the ending of the spawning typically coincides with the onset of the chum salmon spawning and the hungry bulls are quick to take advantage of the food source those chums provide. During most of November and much of December one rarely finds active spawning salmon in the Skagit or Sauk without at least a few bull trout nearby. Even chums spawning in out of the way locations attract these opportunistic bulls. It could be that the bull trout scent the ovarian fluids released from the spawning females and follow it to the source. Once behind the spawning salmon the bulls are quick to grab any dislodged insects, sculpins, loose eggs, etc. Other food sources during this critical winter period include salmon flesh from the carcasses, numerous fish species, and other bite size critters.

bull trout

Photo by @Josh

In addition to the bull trout’s amazing diversity in behaviors they are also equipped to opportunistically take advantage of feeding bonanzas. In the fall of 1995, the Skagit experienced a large pink salmon escapement that also happened to coincide with stable flow conditions during their spawn. This set the table for bulls as they arrived in early November along the main Skagit looking for feeding opportunities. The pink escapement was large enough that second and third waves of spawners dug up the preceding waves of spawner ‘s redds dislodging an astounding number of loose eggs. In most of the large pools the eddy bottoms were covered in loose eggs (looking as if someone had dumped buckets of eggs in every pool. The bulls went on a feeding binge lasting until a high water flushed the remaining eggs downstream. In that 2 to 3 week feeding spree many of the fish more than doubled their weight with some of the 26-to-28-inch fish weighing up to 10#. The fish were bloated with huge girths and had a soft squishy feel. They had clearly stored that huge food intake in what appeared to be fat deposits. The interesting part of the story is that over the next 3 to 4 months those bulls were able to convert that stored fat into growth. Some of the 20-inch fish gained 3 or more inches in that relatively short period while reverting to a more “normal” body condition. Unfortunately for the bulls such egg bonanzas rarely occur.

The winter of 2001 provided a different foraging opportunity. Typically, the various salmon carcasses are washed out of the river by high waters limiting their availability to the forage bull trout. That year there were below average flows with no significant floods all winter. Those favorable conditions resulted in the chum carcasses and the associated flesh being available to the bulls all winter. Salmon flesh was the dominant food item through February. Nearly every bull caught reeked of rotten flesh and if an angler was not careful the bull might regurgitate on them; I leave you to imagine the smell of 3 month old salmon flesh. At the end of the winter the fish were in excellent shape. Once again, those conditions are rare.

In addition to actively feeding the post spawn bulls typically work their way back into the habitats used in their earlier life histories. The anadromous fish move downstream at a variable rate stopping regularly to take advantage of potential feeding opportunities with those fish starting make the transition back into the salt in late January through early April. Some seem to have a hard time deciding whether they want to be in salt or freshwater with some moving back into freshwater in either the Skagit or other nearby river basins. This back-and-forth movement could be driven by forage opportunities. Though by March there always seems to be at least some bulls in the marine waters actively foraging on the same fish species as in earlier in their lives. The fluvial fish also basically repeat the same downstream migration as when they were juveniles. In fact, many end up in the same “home” pool.

skagit bull

Photo by @Leland Miyawaki

In one tagging study where bull trout caught on hook and line (single barbless hooks), tagged with a numbered floy tag (insert under the dorsal fin), the date, location of capture, and length as well any other relevant information were recorded. A total of 892 fish (thought to be largely maturing resident fish) were tagged. Over the next several years 134 different fish were recovered with a readable tag number at least once with a number recovered twice and two 3 times. Of those recoveries one was the next day but most were recovered months to years ( up to 3 years) post tagging. Perhaps indicating that those fish were not as catchable as one would think given their reputation as being ready biters would indicate. Growth between captures of those fish that had an essential full growing season was variable. Some fish grew hardly any over a year while others grew as much as 4 inches with the typical being 2 to 2.5 inches/year. Many of the fish recovered had complex spawning migrations moving downstream before reversing direction to enter tributaries moving upstream to spawning while other fish move steadily upstream from the main river to the spawning tributaries. But perhaps the most interesting behavior documented with the tag recovers was the use of a “home pool” for those fluvial bulls. In the period from about 3 months post spawn until the start of the next spawning migration nearly 2/3 of the individual tag recoveries were from the same pool area in which they were originally tagged even though they had the option of choosing from dozens of pools. Several fish were recovered in the same pool in successive years, and one recovered 3 years after the initial tagging in that original pool. Radio tagging studies in other areas have also found that tendency of bull trout establishing and returning to such “home pools”.

Once the fish reach maturity, they spawn annually with a relatively high portion of any given year’s returns being fish that had previously spawned; some having spawned 4 or more times. Often 50% of the returning adults will have previously spawned at least once. Having several brood years (different birth years) provides stability to the spawning population. In the case of a weak year class (due to flooding or droughts for example) there are other year classes in the population to help fill in any voids. As noted in the spawning chapter the larger/older females have significantly more eggs. This leads to a situation where these repeat spawners typically have a potentially greater egg deposition than the more numerous first-time spawners. For those repeat spawners that are over 30 inches, more than 95% will be males and all of the largest individuals will be males.

While it is tempting to place bull trout in nice "neat" life history boxes, the observation of mixed life histories interacting on the spawning grounds punch holes in the side walls of those boxes. After a period in the salt those anadromous bulls have a predictable phenotype (observable characteristics); that is, they have a silvery color, pale spots and a robust body shape. At the same time the fluvial bull exhibits a phenotype with more greenish/dark back color, spots of more intense color and a sleeker body shape. As the fish near spawning those phenotypes merge. Over two years taking advantage of these phenotype differences during the early portion of the spawning migration scale samples were collected from a number of fish noting their current phenotype (fluvial or anadromous) and total length. Of those sampled 152 (89 with the anadromous phenotype) had spawned at least once previously and had readable scales allowing an examination of the previous growth patterns. Of those at least 8 or about 5% had changed from one life history at first spawning to another when sampled. Based on growth patterns it appeared that 3 of the 8 had switched from a resident to fluvial life history, one from resident to anadromous, 3 from fluvial to anadromous, and one from anadromous to fluvial. Three interesting examples of those life history switches include: a 22.5-inch fluvial fish that was in its 9th year and had reached first reached maturity as a 9-inch fish (headwater resident). A 30.5-inch anadromous fish was in its 11th year of its life (on its 8th spawning run) and had first spawned at about 9 inches (headwater resident). And a 27.5-inch fluvial fish in its 8 year that first spawned at 20 inches (anadromous fish).

Bottom line, the diversity in life histories, the ability to adapt to changing forage and life histories, their longevity, and high fecundity at larger sizes provide bull with significant hedges against adverse conditions and contributes to population stability.

northsound salt bull

Salty Northsound bull photo by @MarshRat