Good Luck and Thank You
It’s been crazy hot these last couple of weeks at San Luis Reservoir. The heat has been so intense that even Dan Blanton canceled a trip due to concerns about heat stroke out on the lake. This prolonged hot weather has made it one of the longest heatwaves on record for the reservoir. Two weeks ago, the surface water temperature was near 80 degrees. This time of year, I start to worry about the survival rate of striped bass released back into San Luis Reservoir.
Algae Blooms in the Bay of Pigs can either increase dissolved o2 when they bloom (through photosynthesis) or decrease it when it decomposes. Algae blooms can cause a spike in DO during daylight hours. For example, a dense bloom could increase DO levels from 5 mg/L to 15 mg/L in the surface layers which may explain why I do better fishing fresh matts of Algae along the shoreline. When a bloom collapses, known as Post Bloom Collapse, the decomposition process can consume oxygen rapidly. This can decrease DO levels by 5-10 mg/L or more, potentially creating hypoxic or anoxic conditions, especially in deeper waters where mixing is limited.
The Algae Oxygen pump
Most California anglers know that we lost a great local fishing publication last October, California Fly Fisher. I’m happy to announce that it has been relaunched, and I highly recommend it to all California fly fishermen as one of the best sources of local fly fishing information. In the inaugural summer issue, I read an article by Michael Malekos on summer fly fishing for trout, which got me thinking about my home waters at San Luis Reservoir. The unique striper habitat of San Luis Reservoir and O’Neill Forebay presents specific threats to the mortality of released fish, mainly related to dissolved oxygen levels in the water. Unlike ocean and Delta stripers, water temperature and dissolved oxygen levels vary more in our landlocked freshwater habitat than in any other California striper habitat. Northern California coastal ocean temperatures range from 50-61°F throughout the year, whereas surface water temperatures in San Luis Reservoir can range from 45°F to 82°F. Last week, the surface water temperatures at San Luis were near 80 degrees. The largest fish I ever caught on a fly, a 35-pound hen, died after I unsuccessfully tried to resuscitate it for an hour, pulling it through deeper, colder water. That fish never had a chance with the water temperatures in the low 80s and being caught in 25 feet of water.
According to the U.S. Fish and Wildlife Service’s “Habitat Suitability Index Models: Striped Bass” by C.R. Tomasso, to ensure sufficient dissolved oxygen for the survival of striped bass, water temperatures should ideally be maintained below 68°F. Striped bass require a minimum dissolved oxygen concentration of about 5 mg/L to sustain their health. However, as water temperature increases, the amount of dissolved oxygen that water can hold decreases, which can be critical for the survival of striped bass.
Several variables affect the mortality of released stripers. Freshwater holds more dissolved oxygen than saltwater. Wind and current can also increase the amount of dissolved oxygen, while algae blooms can precipitously drop oxygen levels at any water temperature. Additionally, playing a fish too long or holding them out of the water for too long can also increase the minimum oxygen needed to survive after release into warm water.
Breath
While a striped bass swimming off on its own generally indicates a lower immediate mortality rate, the overall mortality (including delayed effects) remains influenced by several factors. Even for fish that appear to swim away strongly, the mortality rate can still be within the 5% to 15% range, accounting for both immediate and delayed mortality due to stress and injuries. Proper handling, cooler water temperatures, and minimizing injuries can help improve the chances of survival for released striped bass.
In a recent study, “Assessing Impacts of Catch and Release Practices on Striped Bass” (John Tiedemann, Assistant Dean, Monmouth University School of Science, Dr. Andy Danylchuk, Assistant Professor, University of Massachusetts Amherst, July 2012), the mortality rate for all hook-caught stripers was estimated at 8%. The study also estimates that freshwater mortality is higher due to higher water temperatures. Many studies find that the mortality rate in stripers increases substantially when water temperatures exceed 70°F because the warmer the water, the longer it takes for a striper to recover from a fight or injury post-release. Additionally, ambient air temperature increases stress-induced mortality, especially where there is a large difference between water and air temperatures. Releasing a striper in San Luis during the heat of the day has a higher mortality rate than catching one in the same water temperature in the morning. We must minimize fish time out of the water. Another study showed that freshwater stripers experience more stress during release than in saltwater because osmoregulatory dysfunction and electrolyte imbalance are different in saltwater.
The inability of striped bass to recover from physiological stress incurred during capture can disrupt normal feeding patterns, increase vulnerability to predators, reduce their ability to fight off diseases and parasites, and ultimately result in post-release mortality. As a conservation-minded angler, the goal is to use best practices for careful catch and release that reduce stress and minimize injury to striped bass. The best we can do is fight fish quickly, minimize their time out of the water, and try to avoid conditions where water temperatures combined with high ambient temperatures and algae blooms make survival rates too low. Maybe catch two to harvest and leave.
Live long and prosper
