Lake O update from TRM’s blog to remind anglers of the background reasons for DOC’s regulation changes.
NOTE: (Feedback ever since from TRM’s fishy inmates and other anglers does not concur with DOC’s analysis and feedback. This makes for interesting discussions at the Lake O boat ramp… WATCH THIS SPACE!)
Remember to regularly practice the CCD routine at Lake O – Check Clean and dry your gear before arriving at this trophy lake.
Another reminder that the lake will now be closed for February. Several overseas bookings were unaware of the changes and have had to rejiggle their flights to fit in with the new timings.
Following regulation changes followed by the 2020 summary from Taupō fishery scientist Dr Michel Dedual (since ret’d) for Taupō Fishing Club.
Date: 22 June 2022
Regulation changes
Four regulation changes aimed to protect the long-term future of this iconic trout fishery.
- The season at Lake Otamangakau will start a month earlier from 1 September.
- Lake Otamangakau will close to trout fishing during February.
- A maximum takeable size of 55 cm for trout will be introduced. There will be no minimum takeable size.
- The bag limit will be reduced to two trout.
DOC Operations Manager Taupō Fishery Dave Conley said, “We are pleased to see support for these important changes which we firmly believe are needed to protect the future of Lake Otamangakau, particularly during the hottest time of the year where elevated water temperatures pose a threat to trout.
“Our technical team, led by Fishery Scientist Dr Michel Dedual, carried out a robust investigation over a two-year period which pulled together data from multiple studies, as well as listening to feedback from anglers. The result was a clear set of proposals backed by solid scientific evidence.”
(Feedback ever since from TRM’s fishy inmates and other anglers does not concur with DOC’s feedback. This makes for interesting discussions at the Lake O boat ramp… watch this space!)
Why change?
In answer to anglers’ questions and scepticism during the last three-four years, the 2021 scientific work was triggered by increasing feedback claimed from anglers (?) who reported seeing dead trout in February, when water temperature is at its highest. DOC advise they were also informed by the ongoing debate within the angling community around encouraging trophy trout at Lake Otamangakau.
Changes to the fishing season are designed to help protect large trout when they are vulnerable to heat stress. Catch-and-release is a popular approach used by anglers at Lake Otamangakau. The warm water phase of our research confirmed trout only had a 60 percent survival rate when released in these conditions, therefore stopping fishing during the hottest month of the year will reduce unnecessary trout mortality.
The new maximum takeable size and reduced bag limit will help protect larger trout. Spawning trap data confirms trout at Lake Otamangakau continue to grow beyond their maiden spawning year. In simple terms, the longer they live the bigger they get. Protecting larger specimens will allow them to grow even bigger and increase the opportunity for anglers to catch fish over 10 lbs (4.5 kgs).
Below is the previous report by DOC to help explain the reasoning behind the changes to the rules:
This report by Taupō fishery scientist Dr Michel Dedual summarises a body of research and sets out potential management options for the future of this iconic trout fishery.
Date: 01 December 2021
DOC’s Report title: “Management of Lake Otamangakau fishery in the face of various anglers’ aspirations, changing climate and evolving societal perception of recreational fishing.“
Introduction
The main objective of this study is to apply a decision-making process to support possible changes to fishing regulations during extreme environmental conditions. Twenty-seven years of research and monitoring provides us with a very good understanding of trout populations at Lake Otamangakau (Lake O), but we must also consider the following elements:
- Angler surveys indicate the fishing community is split between two distinct groups of anglers; those who want to catch less but larger fish and those who want to catch as many fish as they can. This dichotomy can create substantial difficulty for managers as any decision may affect one group more than the other.
- The number of trophy fish (herein >650 mm) has declined steadily between 1994 and 2014 and it is now very low, probably no more than 100 specimens.
- Trophy fish at Lake O are large not because they grow fast but because they live a long time. To reach a trophy size they must be at least 5 to 6 years old, meaning that they grew and survived fishing pressure over a longer period.
- The fishing mortality includes fish that are killed voluntarily (harvested) and those that are killed involuntarily by releasing them after capture (catch-and-release).
- Fishing pressure is “the elephant in the room” affecting mortality. Even in the absence of harvest, mortality caused by catch-and-release may be sufficient to eradicate trophy fish if fishing pressure is sufficient. Fishing pressure (visits/year/hectare) estimated in Lake O is amongst the highest in New Zealand.
- Mortality associated with catch-and-release has been well studied worldwide but results are difficult to extrapolate as they are sometimes contradictory and/or specific to a geographic location. However, the major consistent driver of mortality is water temperature within which fish are played – the higher the temperature the higher the mortality. In Lake O catch and release mortality ranges from 0 to 30 % at water temperatures of <11 and >22 ˚C respectively. Furthermore, these estimations of catch-and-release mortality can be underestimated for several reasons. The number of times an individual trout was hooked and released wasn’t considered but with the current level of pressure in the lake, it is reasonable to expect that some fish that survived the first release may not survive an additional catch-and-release event. No data are available on the impact of repeat capture, but some studies suggest that if a trout had a 30 % chance of mortality after one capture event, the total probability of mortality after 2 hooking events could increase to 50 %. We don’t have any data on the number of times the same trout can be captured during its life. Mortality beyond 66 % cannot be ruled out, as several studies have shown that fish vulnerability to predation or susceptibility to disease can increase following catch-and-release.
- Elevated trout mortality at high water temperatures has pushed some fishery managers to close waters to recreational fishing when temperatures climb above a certain threshold. In most cases at 18 to 20 ˚C.
To explore what type of management action could be the most effective at maintaining anglers’ satisfaction levels and fish population perennity, we first used the biological aspects of trout runs collected over 20 years of trapping to build a dynamics model. We then submit this population to various degrees of fishing pressure, water temperature, changes caused by global warming and the corresponding catch-and-release mortality, into the model. The impacts of global warming were considered using 23 management regulation scenarios. We estimate these impacts on the catch rate of all fish, trophy fish and the duration of a fishing season. The outcomes of each scenario are presented in the table below.
Table 1. Outcomes and impacts of various fishing regulations for Lake Otamangakau on CPUE (fish/visit) of all sizes and trophy fish (>650 mm) and implications on the duration of the fishing season. | View larger (JPEG, 200K)
Results and discussion
The highest Catch Per Unit Effort (CPUE) would be expected with regulations allowing only catch-and-release combined with a closure of the fishery when the water temperature reaches 11 ˚C. This temperature threshold represents the condition when we observed no mortality caused by catch-and-release (commonly called “no kill”). However, such a regulation would dramatically reduce the fishing season restricting it to just over a month (4.7 weeks). The lake surface water temperature remains below 11 ˚C from April to November although this period will diminish with global warming.
Some anglers may suggest extending the fishing season to include the entire duration with water <11 ˚C. I think this would add another unnecessary stress on fish migrating to spawn and during the early phase of their recovery.
Note: For clarity and conciseness, I didn’t put the impact values of the increases in water temperature caused by global warming but considered scenarios with different rates of temperature increase per year. The results indicate that the rate of temperature change will negatively affect both the CPUE and the duration of the fishing season.
At the other end of the spectrum maintaining the status quo (doing nothing) or having a compulsory harvest of fish >350 mm in February would generate the lowest CPUE but would maximise the duration of the fishing season.
We believe that neither of these two options are desirable nor justified. We know that the number of fish in Lake Otamangakau has a negative impact on their average size, so aiming at no mortality (catch-and-release only and closure at 11 ˚C) could lead to an overpopulation of fish that would prevent the production of trophy fish and hence negate the benefit of a change in fishing regulation. On the other hand, not changing anything or relaxing harvest further could potentially reduce the fish population to levels where no trophy fish are available.
As we said earlier, there are several types of anglers fishing Lake O, with some who want to harvest fish for the table. We have analysed the length class distribution of the fish kept by anglers and we can see that the number of fish kept is small and that the average size is less than 550 mm indicating that anglers are not selecting trophy fish to harvest (Graph 1).
Graph 1. Length class distribution of the fish kept (n=269) by anglers recorded during angling survey (n=2343) carried out at Lake Otamangakau between 1997 and 2020. | View larger (JPEG, 121K)
Therefore, we are confident that allowing some harvest will not have a significant detrimental effect on the availability of trophy fish and that a catch-and-release only regulation is not warranted. However, even though the surveys indicate that virtually no angler seeks to harvest the allowed daily bag, I would suggest that reducing the daily bag to 2 fish would send the right message.
In fact, the outcomes of catch-and-release only without any temperature threshold closure will have a less positive impact on CPUE than allowing some harvest of fish with some size regulation and water temperature restriction.
Setting the threshold temperature at 20 ˚C for closing the fishery would have the best outcome but it presents some logistic problems and can have an unfair impact on anglers. The temperature in the lake can change quickly from one day to the next so it would be difficult to put in place and enforce such a rule. Furthermore, it would be unfair to those anglers who plan trips to Lake O at great expense, only to arrive and be prevented from fishing because the water temperature is over the threshold.
The continuous temperature monitoring indicates that the lake surface water temperature is highest from mid-January to mid-March. So having a fixed closure period covering February would be simpler to understand and easier to enforce.
Considering all the limitations and constraints of the various scenarios, I believe that having a maximum size of 550 mm combined with a February closure constitutes the best compromise to address the angler’s satisfaction and maintaining and protecting the trout population.
Even though the differences in CPUE outcomes between the different scenarios seem very small we must remember that the total fish mortality will be multiplied by the fishing pressure. The Lake O fishery is open access; therefore, the long-term effectiveness of the above regulation will be dependent on the unknown and unpredictable fishing pressure exerted.
Restrictions on angler effort may represent an important management option for maintaining the trout population. However, policies that seek to reduce fishing effort are often unacceptable and hence have rarely been implemented in recreational fisheries. In addition, reducing fishing effort will also mean less revenue from licence sales – the only source of income. Therefore, before reducing fishing pressure managers need to have a clear understanding of the financial impact, particularly when angling participation is in decline and managers need to retain current angler participation as well as recruit new anglers.
Summary of a presentation from Taupō fishery scientist Dr Michel Dedual for Taupō Fishing Club.
Date: 28 August 2020
Background
Dr Dedual was invited by the Taupō Fishing Club in July 2020 to speak about recent research findings, including a major piece of work focused on Lake Otamangakau (Lake O).
Recent angler reports of dead trout sighted at Lake O during hot, dry summer conditions was a real concern for the Taupō fishery team. We know from research carried out by international colleagues that warm water with low dissolved oxygen is a lethal combination for trout (>20ᵒC is the key figure).
Such warm conditions are not unusual at the surface of Lake O, particularly during dry hot summers when cool water flowing into the lake from the western diversion of the Tongariro Power Scheme is reduced. However, water temperature is not uniform throughout the water column. Cooler temperatures are found in deeper parts of the lake which normally provide sanctuary for trout, even during extended hot weather. So, what is causing the recent increase in trout deaths and what can we do about it?
Investigation
To better understand exactly what is going on at Lake O we introduced monitoring equipment to record temperature and dissolved oxygen levels at varying depths and locations within the lake. In addition, we set up an automated monitoring system to quantify angling pressure. Finally, we ran a ‘catch-and-release’ survival study to capture the impact of this increasingly popular fishing approach. The results from this work were also combined with results from regular ongoing angler surveys and previous spawning trap data.
Previous research
Before we consider the results from recent studies it’s valuable to look back at previous work and consider what we already know.
Ongoing angler surveys tell us that voluntary catch-and-release has become increasingly popular, particularly at Lake O. In recent years between 80% and 90% of anglers adopted this approach at Lake O compared to an average of 60% to 70% in Taupō rivers and approximately 30% in Lake Taupō.
From spawning trap data gathered between 1995 and 2013 we know the trout population size in Lake O has increased significantly over time. Over the same period the number of very large trout decreased. For example, in 1996 there were 92 fish over 4kg recorded in the fish trap – which equates to 12% of the spawning run. By 2009 that figure was down to 1 fish over 4kgs and 0.03% of the run. As expected, the increased trout population resulted in a higher catch rate for anglers but the chances of hooking a very large fish (over 4kgs) diminished.
Key results from the study
Our latest studies confirmed Lake O receives very heavy angling pressure – from October 2019 to March 2020 it received 2,487 visits representing 13.8 angler days/hectare. As a comparison Lake Taupō is estimated to receive only 1.1 angler days/hectare. The distribution of angling pressure at Lake O is also a concern, with the largest concentration of angler visits coinciding with the warmest weather in January.
The catch-and-release investigation gave a clear indication that warm surface water has a considerable impact on trout survival rates at Lake O. The early season sample, which was collected during cooler water conditions in November, delivered a 100% trout survival rate. The later sample collected during hot summer conditions in February/March revealed a mortality rate of around 30%. The graph (figure 1) captures this information along with key environmental conditions.
Summary of Lake Otamangakau mortality research data – view larger (JPEG, 157K)
Fishery management
Any potential changes to the Taupō fishery regulations must be based on robust evidence. The combination of our primary data collected at Lake O and the data generated by other members of the scientific community provides a firm platform for change.
Water temperatures above 20ᵒC and the associated drop in dissolved oxygen levels can easily kill trout. Under these warm conditions catching and releasing a trout at Lake O effectively draws the fish away from the safety of cooler water, into the dangerous conditions found near the surface. Approximately 1 in 3 of these fish will die. High angler pressure encountered during warm water conditions in January and February adds to the problem.
Some form of restriction on fishing is required to protect trout at Lake O when surface water temperature exceeds 20ᵒC. We are currently evaluating options. From a technical perspective we are looking at modelling that considers environmental conditions, angling pressure, catch rates, and potential trout mortality. We are also mindful of the angling community and want to ensure any changes are fair and enhance the long-term recreational fishing opportunities at this venue.
A draft management plan for Lake Otamangakau will be generated and shared with the Taupō Fishery Advisory Committee (TFAC) – made up of representatives from a cross section of angler interest groups and fishery stakeholders. Feedback will be sought in response to the draft management plan.
GENERAL INFO: (Amended)
Despite its small size, Lake Otamangakau is known for 2 things, the incredible size of trophy trout and the increased number of trout in the wild fishery. Some trophy trout caught there were recorded as big as 4.5 kg, but even on a “slow” day, you can expect an average weight of 2-3 kg. Lake Otamangakau is a shallow hydro-lake formed in 1971, seated near the iconic volcanic range of the Tongariro National Park, just south of Turangi. The volcanic ranges and snow-capped peaks of Mount Tongariro and Mount Ruapehu seem so close you can reach out and touch them from your boat.
On a fateful day in 1983, a local Taupo fishing guide caught a record 8-kg trophy brown trout, cementing Lake Otamangakau as a wild trophy trout fishery, famous around the world. With the perfect storm of ideal conditions like current flow, cold water temperatures, and oxygen levels, trout thrive and return season after season to spawn here.
The fishing season for Lake O begins from 1 SEPTEMBER through to 31 May (less February!). But some of the best fishing is late January when there’re plenty of cicadas for trout to feast on. If you’re going to catch and release, your best chance is to fish on either side of peak summer when the waters are cooler for fish to surface.
Because the lake is so shallow and with two boat access ramps, it makes it a premium spot to fish from a small pontoon boat, jigging or casting your fly in the most remote weed beds and banks.
