Pond Boss Forums INTRO Questions & Observations What makes exceptional trophy lakes different?

I always wonder what makes lakes like Dixon Lake, CA and Lake Biwa, Japan different than others. Usually, when you read about those differences that make trophy lakes exceptional the explanation is a particular forage fish that flourishes there. To be sure, consumption of forage is essential to growth but I wanted to explore other possible factors that raise the bar for the best of the best when it comes to ultimate size of LMB. Both Dixon and Biwa have mixed Florida and Northern LMB genetics and both have produced LMB breaching 20 lbs. In the case of Dixon multiple fish greater than 20lbs have been caught to include the LMB fondly called Dottie which was last caught in 2006 at a weight exceeding 25 lbs. Dottie was foul hooked on her last appearance and so was not submitted for a record. In Dixon, forage is supplemented (unintentionally) by RBT stocking. We know that in both lakes the LMB are getting plenty to eat because they grow to 20+ lbs. So I wonder, what makes them different than other lakes that produce similar or larger amounts of forage yet are not able to produce such impressive fish.

Water quality is a potential factor and so I have investigated this as a potential contributor. One factor of water quality that interests me is temperature. I have recently formulated temperature dependent conversion parameters from the findings of the paper on LMB conversion of GAM that I referenced in the thread titled "LMB are remarkable converters of fish". There are two parameters affecting conversion. FCR (food conversion factor). One is Intrinsic FCR. This is the FCR of the consumption exceeding what the LMB needs for metabolism. So this is not Gross FCR. The other factor is Specific Maintenance Rate (SMR). The SMR is the weight of forage that is needed to maintain the weight of an LMB. It is weight of prey divided by weight of predator and it applies to all LMB regardless of their actual individual weight. As one might suspect, these parameters are temperature dependent and so using a single value for each is a rather blunt instrument. Applying the temperature dependency provides a richer understanding for the seasonal forage requirements for maintenance and growth. Depicted in the graphs below, the Intrinsic FCR improves with increasing temps (a lower number reflecting more efficient conversion) while the SMR increases with increasing temps. Higher SMR works against efficient conversion. So the two parameters work against the other with regard to temperature. Low SMR helps at low temps while High FCR hurts at low temp. The inverse applies to high temps.



It turns out that the temperature that is optimum for the most efficient conversion depends on the consumption above maintenance. Since growth is a function of consumption above maintenance, we can alternatively say that the optimum temperature is related to the rate of growth. By holding the growth rate constant we can divide the consumption rate required for the specific growth by the specific growth rate and the result is the Gross FCR (the FCR after the maintenance requirement has taken its share of the consumption). Assuming a fixed daily growth rate we can then calculate the Gross FCR across the spectrum of temperatures to find which temperature is most optimum. For a fish that is growing on any given day at an annualized rate of 20% the optimum temperature is predicted to be a frigid 39.2 F as depicted in the image below.



This finding wasn't something I had anticipated. But this daily consumption to support that growth is only very marginally above what is needed for maintenance and because what is needed for maintenance is so much less in frigid water ... the FCR of 20% annualized growth is optimum at that temperature. For BG prey ... the Gross FCR is 15 when growing 20% annualized at 39.2 F where it must consume 3/4 of 1% of its body weight daily. Big fish grow this slow or slower but smaller fish can grow much faster and if they are growing fast enough the optimum temperature will be higher than 39.2 F. At growth rates exceeding 90% annualized growth ... the optimum temperature increases. Obviously, a 10 lb LMB can't double its weight in a year and so we can come away with this insight. For fish growing at slower rates,(that is big fish), cooler water favors their growth over warmer water ... provided ... the same quantity of prey is available and consumed.

Working with daily average temperatures I found that I could construct formulas which model the average daily temperature as a function of the day of the year. I wanted to compare DFW (Dallas Fort Worth) with Dixon Lake (Escondido, CA) to see if there exists any advantage to Dixon Lake temperatures compared to DFW. The temperature profiles are depicted in the leftmost graph below. Note how the average temps in Escondido CA are much more moderate than DFW temps where the temps are much cooler in the summer and only modestly warmer in the winter. On the right side are Dixon and DFW SMR curves reflecting the differences imposed by temperature. It is clear that it takes more consumption to meet maintenance requirements in DFW than it does in Escondido.



One way to quantify the difference temperature makes is to constrain one location to maintenance consumption. Start each with the same weight of fish and then allow them to consume equal portions every day based on the maintenance consumption of DFW. Below is the comparison where Dixon LMB can gain 14% on DFW consumption that leads to no gain.



Now I will mention that the advantage is temporary without mortality. In other words, the temperature advantage actually only allows for an increase in carrying capacity. Once the standing weight in Escondido reaches carrying capacity... just like in DFW ... there is no growth. This can be demonstrated by simulating a second year where Dixon starts with a standing weight 14% higher than DFW. This result is depicted below where the Dixon standing weight finishes the year only marginally better. The effect year after year diminishes terminating at a 17% increase in carrying capacity. Alternatively on can calculate how much maintenance is required for the same standing weight of fish ... the result is ~17% less than in DFW. This demonstrates Swingle's principle that fish grow into maintenance and then stop growing (without mortality). To maintain the benefit of temperature in fish growth year after year requires balancing the benefit with mortality that is essentially equal. IOWs if Dixon has 14% greater mortality than DFW then the temperature benefit will be realized (evidenced in the growth rates of individuals).



So we can draw a couple of conclusions from this thus far. The temperature profile of Escondido, CA is favorable to the growth of large LMB AND the mortality of large LMB is also high enough to keep them growing throughout their lives. It makes sense that Dixon has high mortality of large LMB in that it is a trophy destination that gets a lot of fishing pressure. Per acre, more fishing man hours than most impoundments. This small (68 acre) BOW has nonetheless produced many trophy LMB. At Biwa Lake in Japan, it is illegal to release an LMB unharmed. They must be removed and killed. So the mortality in Biwa is probably much greater than in Dixon (as a % of biomass). Biwa is another favorable temperature profile for large LMB, with high mortality, and without the same benefit of supplemental trout forage.

I will also mention that both Biwa and Dixon have clear water. Clear water facilitates the use of deeper cooler water as temperature refuge for big fish by letting light penetrate and generate O2 production. Big fish use this water to lower their the metabolic requirement for consumption and to better convert what they consume to growth. Low clarity will tend to reduce a large LMB's ability to use cooler water because the O2 production below light penetration will cease.