Thank you to so many TRM readers for the “likes” and messages following photos of Robbie Cadogan’s 11 3/4-pound Rainbow beast from Lake O. This raised more questions with inmates comparing Lake O to the mighty Tongariro River. In answer to questions, we repeat DOC’s trap results from the Te Whaiau trap last August. This is prompted after failing to spot a single angler in all the town pools last Saturday, compared to Lake O’s launching ramp trailer count of 14!
(SWMBO’s hidden agenda is to encourage more fishos from West Island to visit TRM instead of Twizel!)
From Fishery Focus Issue 42 dated August 2025:
Lake Otamangakau opens.
… Lake Otamangakau is a small, shallow hydro-lake with a big reputation for producing large wild trout – trophy fish over 10 pounds (4.5kgs) in weight are caught every year… With the season opening on a Monday, there will probably be far fewer anglers around, providing a bonus for those who are able to make it. As an additional bonus, this edition of the newsletter contains a summary of the latest trapping results from the Te Whaiau Stream.
Te Whaiau trap summary.
Lake Otamangakau opens to fishing next week, so this is a good time to review the results from the winter trapping programme on the main spawning tributary flowing into the lake. Fish trapping on the Te Whaiau Stream began in late May and ran through to late July. This is the 27th season that spawning data has been collected from this location. The stream was continuously trapped for 23 years, from 1994 to 2016, before being decommissioned. Trapping resumed in 2022 following significant upgrades to trapping infrastructure, enhancing fish safety and reducing operating costs. The trap provides ongoing monitoring data on the mature spawning population and allows us track population changes over time.
Number of trout trapped.
During late May, a total of 49 rainbows and 115 browns were trapped and processed by staff in the upstream pen (Table 1). In June, a further 184 rainbows and 129 browns were also trapped, followed by another 84 rainbows and 2 browns in July, taking the actual total to 317 rainbows and 246 browns over the 10-week period. Each fish was sexed, measured, weighed, fin clipped, maturity estimated and then released upstream to continue spawning.
To allow the two-month trapping period of recent years to be comparable with the five-month period from earlier years, formulas are used. Separate formulas were created for the browns and rainbows based upon the historical trapping data (1994-2013). The June-July period alone accounts for more than 95% and 88% variations in the total run of rainbow and brown, respectively. For rainbows, the formula is 1.42*(June + July actual count) +57.46 and for browns; 1.12x(June + July actual count) +445. With these formulas applied, this season’s April to August run estimate for the browns increases to 592 trout and the rainbow trout run increases to 438 (Fig.1). Of note, is the longest average length on record for the rainbows during 2025 of 619mm (618mm peak during 2023) but the 3rd smallest run total behind the very early years of 1994-1996 further indicating a smaller population but bigger fish. Similarly, the browns were the second longest on record at 612mm (peak of 617mm in 2023) and this winter saw the ninth smallest run recorded behind the early years of 1994 & 1995 and more recent years.
The trends in abundances of rainbow and brown trout in the spawning runs remained similar until the mid-2000’s, when they started to show marked differences. The spawning rainbow trout population steadily increased from 356 trout in 1994 to 1,439 in 2005 but then surged after 2006 to reach a peak of 3,661 trout in 2009. The rainbow trout spawning population then steadily declined towards 2016 returning to a level similar to that of the mid-2000’s. The trap was not operated during the period 2017-2021 but when reinstated during 2022, the spawning rainbow trout run had returned to a level not seen since the mid to late 1990’s. The abundance of brown trout also steadily increased from 329 in 1994 to 1,262 in 2008 and then decreased, but unlike rainbow trout, did not show any sudden changes. The abundance of browns during recent years are still comparable with those from 2016.
Size of trout
The increase in spawning run size was associated with a marked decrease in average weight for both browns and rainbows especially between 2007 and 2013 when the numbers of browns and rainbows exceeded 3,000 trout in total. The increase in spawning biomass was particularly large for rainbow trout which increased from 997kg in 1994 to 7,116kg in 2010 compared to an increase from 790kg to 2,504 kg for brown trout during the same period (Fig.2). The increase in trout abundance was initially caused by an increase in recruitment of maiden fish followed by an increase in the survival of previous spawners. During this time, the average size of the rainbows decreased, indicating that the carrying capacity of the lake may have been reached which had implications for the management of this fishery.
Overall, the browns trapped during late May, June & July 2025, averaged 612mm and 2.8kg (6.2lbs) with an average condition factor of 43.2. These are on par with the browns trapped last season (605mm & 2.8kg). The heaviest brown trapped was a hen measuring 700mm and weighing 4.32kg (9.5lbs) which is down slightly on the heaviest brown trapped last year – another hen at 670mm and 4.45kg (9.8lbs). Since 2022, the average length of browns has ranged between 597mm and 617mm while the average weight has ranged between 2.7 and 3kg. Overall, the browns trapped this winter were the 6th longest and the 5th heaviest on record (Fig.3).
The rainbows trapped during late May through to the end of July this year averaged 619mm and 3kg (6.6lbs) with an average condition factor score of 45.3. Again, these are very similar to those rainbows trapped during last season (617mm & 3kg). The heaviest rainbow trapped this season was a hen measuring 720mm and weighing 5.04kg (11.1lbs). This is down slightly on the 5.55kg (12.1lbs) hen trapped last year. Overall, this year’s rainbows were the longest and the third heaviest on record (Fig.4).
Trophy trout.
A total of 5 trophy sized trout (exceeding 4.54kg or 10lbs) were trapped this winter – all of which were rainbows; 4 males and 1 female. All bar one of these trophy rainbows measured over 700mm in length with an average condition factor of 46.6. This equates to just 1.6% of the total recorded rainbow trout run (Fig.5). It is encouraging to note that there were an additional 16 rainbows were trapped that weighed between 4kg (8.8lbs) and 4.4kg (9.7lbs) which have the potential to become double-figured fish in the next couple of years if they survive. Despite the browns being in the top 5 for weight this season, no trophy sized browns were physically trapped in 2025. However, a total of 7 browns were trapped that weighed between 4kg (8.8lbs) and 4.3kg (9.5lbs) with the potential to reach trophy status over summer.
The percentage of trophy-sized rainbow trout amongst the run has ranged from zero to a peak of 7.9% observed during the winter of 1995 (27 trophy-sized rainbows). 1996 actually produced more trophy sized rainbows with 34 recorded but due to the larger population size that year equated to 7.7% of the total run. 2023 saw the most notable increase in trophy rainbows for 18 years with 8 double-figured rainbows trapped. Historically, the number of trophy-sized browns trapped has never exceeded 2 per season, even when the trap was operated between April and August. Percentages have remained less than 1% of the actual run-up until seasons 2022 and 2023. Overall, the trout population at Lake Otamangakau has remained relatively stable since 2022. The size and condition of trout this year was on par with recent years, indicating that we are dealing with a smaller population of larger fish. With reduced spawning mortality at this venue, these large fish will recover and grow even bigger.