Old News Archive

Rainbow Trout/Brook Trout Study Results

August 13, 2007 - TRC

In the fall of 1997, the Fishery Division established a committee comprised of biologists and hatchery staff to revisit the prospect of a rainbow trout stocking program. After deliberation of the pros and cons, the committee concluded to move forward with a limited, experimental program to evaluate the relative performance of rainbow trout, brown trout, and brook trout. While some people contend rainbows would provide angling diversity, our intentions were to determine if rainbows could provide fishery managers with an additional tool to improve fishing opportunities for Maine anglers. The study was conducted over a 5-6 year period in a variety of Maine waters and included three parts: (1) hatchery performance comparisons among all three species, (2) field performance comparisons of browns and rainbows, and (3) field performance comparisons of brookies and rainbows. A federal hatchery in Tennessee provided Eagle Lake Strain rainbow eggs used during the study.

The study is finally winding down, a draft report for part one and two has been completed. The results of the brown trout/rainbow trout field comparison were highlighted in last winter in a similar article. A draft report for the brook trout/rainbow trout comparisons is currently being reviewed, and a final report is expected to be out by this summer. Highlights from the 3rd and final report are presented in the remainder of this article.

This portion of the study evaluated the relative field performance of Eagle Lake strain rainbow trout and Maine Hatchery strain brook trout in four small, trout ponds including: Jaybird Pond (Hiram), Lily Pond (New Gloucester), Long Pond (Denmark), and Overset Pond (Greenwood). Study objectives were: (1) to compare angler catch/harvest rates and examine whether the two trout species differ in their seasonal availability to the angler; (2) to evaluate relative size and growth, (3) to assess survival and carry-over potential; (4) to compare their utilization of the food chain, and (5) to examine trout performance in waters with marginal summer water quality under different levels of competitions/predation.

Catch and Harvest Rates… Across all waters, legal-sized rainbow trout were caught and harvested at rates 2.5 and 3.8 times greater than brook trout, respectively. These results are not at all that surprising, rainbows were slightly larger than the brookies, and a higher proportion of rainbows were of legal-size rainbows at the time of stocking. A comparison of the combined catch of all legal and sublegal fish (all trout/hour) allows for a more standardized comparison, and addresses the size differential issue. Catch rates (all trout/hr) for individual study waters were higher for rainbow trout on three out of the four waters; however, the overall difference across all four waters was relatively small (1.2 times higher). The data suggest that full season catch rates are fairly similar between the two species.

On the other hand, a review of catch rates by early, mid and late season shows that brook trout typically provided slightly better early season angling opportunities, whereas rainbow trout yielded about 2 ½ times higher catch rates during mid and late season period (Figure 1).

Size Quality and Growth… Brook trout produced fisheries of lower size quality than rainbow trout. Brook trout averaged 11.2 inches long and weighed 0.62 pounds, where as rainbows averaged 14.6 inches long and weighed 1.1 pounds across all study waters. This data demonstrates that rainbow trout typically provided better fisheries in terms of size quality. However, the above data does not clarify whether or not the longer lengths and higher weights for rainbow trout are due to better growth, because the mean size differences may simply be a function of the rainbows being stocked at slightly larger sizes and demonstrating better survival.

To answer this question, the increase in growth since stocking (incremental growth) was examined, eliminating the initial size advantage at time of stocking. In addition, incremental growth data was further broken down by month to develop a monthly grow rate. The use of monthly growth rates allows fish that were sampled at different times during the study to be compared. Rainbow trout exhibited better monthly growth rates than brook trout, both in terms of length and weight. Monthly growth for rainbows was approximately 50% greater than for brook trout for both length and weight.

Holdover and Survival….Rainbow trout survival (holdover potential) exceeded brook trout on three out of the four study ponds. The annual survival estimate for rainbow trout was 2.7 times greater that brook trout (14 and 38%). Across all waters, brook trout older than 1+ comprised only 10.0% of our sample compared to 55.1% for rainbow trout. These results indicate rainbow trout are more likely to provide quality and trophy sized trout fishing opportunities than brook trout.

Diets….Fall diets of brook trout and rainbow trout were very similar, and surprisingly Eagle Lake strain rainbow trout did not appear to utilize larger, non-insect type food items (i.e. fish, mollusks, crayfish) anymore than brook trout. On the other hand, rainbow trout exhibited fewer empty stomachs and a higher volume of food/kilogram of trout. This may suggest that rainbow trout are more aggressive feeders, which could account for the higher growth rate observations.

Water Quality and Competition….Although our sample size was limited to only four ponds, collected survival and growth information suggests rainbow trout were more tolerant of competition and/or predation pressures than brook trout. For example, Lily Pond produced good numbers of holdover rainbow trout in the 14-18 inch range, despite heavy competition from largemouth bass, chain pickerel, black crappie, pumpkinseed sunfish, and several other fish species. On the other hand, we observed only a total of three brook trout during the four sampling events conducted between 2001 and 2006. Interestingly, rainbows demonstrated poorer survival to older ages in two ponds with limited water quality (Long and Jaybird Ponds), despite lower level levels of competition than Lily P. This suggests poor to marginal summer water quality conditions may be more limiting to rainbow trout performance than heavy competition.

In conclusion, our initial and most important reason for investigating rainbow trout performance was to explore their potential for improving angling opportunities for coldwater fish, particularly in marginal trout ponds and “put-and-take” stocking programs. Performance results from this study indicate rainbow trout have the potential to produce longer season fishing opportunities, better size quality fisheries, and a limited number of trophy-sized (≥ 18 inches) trout without sacrificing overall catch rates. However, as a trout, they still have their limitations and will only produce longer season “put-and-take” fisheries of slightly larger size quality in waters with extremely marginal water quality. In such cases, a brook trout stocking program may yield the same returns, except over a shorter period of time. On certain marginal waters currently managed for brook trout, the replacement of “put-and-take” brook trout stocking programs with rainbow trout could improve angling opportunities for coldwater sportfish in Maine.

Before the Department adopts a rainbow trout stocking program, all of the associated hatchery and management implications of such a program need to be considered. For example, if a brood stock were developed, additional equipment would be required to manipulate rainbow trout spawning times if a fall spawning strain is preferred. Fishery managers may need to protect spring spawning rainbow trout due to their vulnerability to anglers and poachers. If rainbow trout do not replace existing program, then the largest obstacle to overcome will probably be associated with space constraints in our existing hatchery system.

Perhaps, the most important consideration in initiating a rainbow trout stocking program is their potential to negatively impact native salmonids like brook trout and landlocked salmon. While I agree with Scott and Crossman’s statement in Fishes of Canada, “The rainbow has been one of the more successful, more appreciated, and less potential dangerous of the many attempts to introduce fish to areas beyond its natural range;” it would be irresponsible of the Department to not seriously consider the risks associated with the introduction of a non-native trout species into Maine waters. An important component of this risk assessment is the development of a sound rainbow trout stocking policy to provide assurances that important native fisheries will not be jeopardized by rainbow introductions. In addition, existing Department policies require all new stockings receive Division-wide peer review. This formal process ensures that new stocking proposals are justified. Additionally, existing policies also have an outreach component, which includes a requirement that public input be sought in response to proposed new stockings. Typical venues for this input include this newsletter, weekly fishing reports posted on the Department’s web site and in some local newspapers, sportemen’s forums, and appearances at fish and game clubs and other groups.

-Jim Pellerin, Sebago Region Assistant Regional Fisheries Biologist, Department of Inland Fisheries and Wildlife

NOTE - This article reflects the views of the author and not necessarily those of the TRC Alliance Team.