No abstract

Establishment of nonnative Northern Pike Esox lucius in Pactola Reservoir, South Dakota, has prompted concern among biologists about the influence of this species on the lake's intensively managed salmonid fisheries. Ancedotal information suggests that catch rates of Rainbow Trout Oncorhynchus mykiss have declined while mean size and abundance of Northern Pike has increased, although quantitative information on diet and growth of the Northern Pike population is lacking. To address potential interactions between Northern Pike and Rainbow Trout, we assessed size‐dependent predation by Northern Pike on Rainbow Trout and determined the relative energetic contribution of stocked Rainbow Trout to Northern Pike growth using bioenergetics modeling. Stable isotopes combined with traditional diet analyses revealed that smaller Northern Pike (<600 mm TL) consumed primarily centrarchids and Rainbow Smelt Osmerus mordax, and Rainbow Trout contributed less than 10% to their annual energy consumption. In contrast, larger Northern Pike (≥600 mm TL) consumed primarily Rainbow Trout, which accounted for 56% of their annual energy consumption. Combining estimates of Northern Pike predation with production costs of catchable‐size Rainbow Trout revealed that annual economic losses ranged from US$15,259 to $24,801 per year. Over its lifespan, an age‐10 Northern Pike was estimated to consume ~117 Rainbow Trout worth approximately $340. Thus, Northern Pike predation substantially influences salmonid management initiatives and is likely a primary factor contributing to reduced Rainbow Trout abundance and return to anglers in Pactola Reservoir. Strategies for reducing Northern Pike predation on Rainbow Trout include increasing the size of stocked fish or altering the timing and spatial distribution of stocking events. Received June 5, 2015; accepted November 2, 2015 Published online March 8, 2016

Fish implanted with acoustic transmitters are assumed to behave and grow after stocking similar to untagged fish. In this study, three groups (tagged, sham, and control) of rainbow trout Oncorhynchus mykiss [mean (SD) initial length = 277 (24) mm] range were maintained together in three raceways for 90 days, with each raceway containing 10 tagged, 10 sham, and 10 control fish. The fish in the tagged group were anesthetized and had an inert transmitter inserted via a ventral incision. Fish in the sham group were anesthetized and had an incision without transmitter implantation, while the control group was anesthetized only. In each raceway, trout with the inert transmitters were significantly lighter and shorter than fish from the other two groups at the end of the experiment. However, the reduction in weight, length, and specific growth rate occurred primarily during the first 38 days posttagging, with tagged fish growing at similar rates to the other two groups for the final 52 days of the experiment. Mortality data indicated a survival threshold of 280 mm length in the tagged fish, with 100% survival of the Rainbow Trout greater than 280 mm and only 59.1% survival of trout less than 280 mm. Based on the results of this study, rainbow trout implanted with 9 x 24 mm, 3.6 g acoustic transmitters should be held prior to release for a minimum of 38 days to ensure similar growth rates as untagged conspecifics, and only trout with an initial length greater than 280 mm should be used to maximize survival.

Two hundred and forty-three juvenile rainbow trout Oncorhynchus mykiss ranging from 73 to 119 mm were implanted with 8, 9, or 10 mm Passive Integrated Transponder (PIT) tags, with tag retention and fish survival observed for 343 days. Similarly, 173 brown trout Salmo trutta ranging from 71 to 86 mm were implanted with 8 or 10 mm PIT tags and observed for 293 days. Over the entire duration of the study, rainbow trout ejected only six tags, while no tags were ejected from the brown trout. All of the rainbow trout tag ejections occurred before 50 days post-tagging. There was no significant difference in the length or weight of fish that ejected tags compared to those that retained tags. No fish died during the trials. These results indicate that rainbow trout and brown trout at the sizes used in this study can be safely implanted with, and subsequently retain, up to 10 mm PIT tags for durations of nearly one year.

We examined the effects of anatomical placement and fish TL on 8‐month retention of standard‐sized (1.2 × 2.7 mm) visible implant alpha (VIA) tags in 255 Rainbow Trout Oncorhynchus mykiss (128–368 mm TL) at 14‐d intervals in a hatchery setting. Anatomical placement was strongly associated with tag retention at each sampling period following day 154, culminating with significantly fewer (P < 0.05) VIA tags retained in anal fin tissue (25%) than in postorbital adipose tissue (55%) on day 224. Logistic regression models suggested that TL was more strongly associated with retention when VIA tags were placed in postorbital adipose tissue than in anal fins. Visible implant alpha tag retention was significantly associated with TL for placement in the postorbital adipose tissue at each 14‐d interval. Tag retention in anal fin tissue was associated with TL during the first 6‐month period of the study and became less significant after day 182. Overall tag retention rates were lowest for fish < 200 mm TL at 15% and 25% for anal fin and postorbital adipose placement, respectively, but rose to 28% and 63% for fish 200–300 mm TL and 30% and 75% for fish >300 mm TL. Due to the effects of fish size and tagging location on tag retention in Rainbow Trout, we recommend VIA tag use only in fish > 200 mm, with placement in postorbital tissue. Estimates of VIA tag retention should also be considered before or during marking studies involving Rainbow Trout. Received January 8, 2014; accepted June 4, 2014