Those who have lived down here for most of their lives will remember the historic freezes of 1983 and 1989. I believe that 1983 has been the most serious freeze in my lifetime, when nearly the entire bay froze over, and the tall fan palms and most of the citrus tree population, died. Following 1983, big trout were few and far between. But they repopulated quickly, and it only takes 6-8 years or so before a female trout reaches 27-28 inches. Then, again in 1989 that bay froze in places, and huge numbers of trout came to the surface, stunned and dying along shorelines, asphyxiating from the low oxygen context of the freezing saltwater. Unconscionable people gathered to snag, hook or net them in their most vulnerable state. Game wardens intervened to keep opportunistic SPI guides from making multiple runs to areas filled with the dying fish.
The freeze we just experienced--with nighttime temps in the low-to-mid 20s--and will suffer again tonight to a lesser extent, may not have had the devastating impact on the lower Laguna that it apparently has had on the waters around Rockport. Pictures of shorelines covered with big trout have been showing up among anglers who have made early visits to the bay's edge. And yet, the rumors among guides and serious fly fishers 100 miles south in the Rio Grande Valley are cautiously optimistic, as reported by my son Ryan, who always has his ear to ground, and networks with several area guides.
From the warmth of my arm chair (after going without electric heat for two days!), I have nothing definitive to say. But the mortality levels among trout, and usually to a much lesser extent among relish, are starting from historically high populations of both species. Indeed, the big trout population has had 32 years to prosper to unprecedented levels of larger fish since the last serious or "hard" freezes of 1989, defined by subfreezing temperatures over a long period of time. A flash frost normally poses no threat. But a sustained 8-12-hour period of subfreezing temperatures quickly transforms the bay from a normally temperature tolerant ecosystem, to a body of water that eventually "catches up" with the air temperature. The available oxygen in the water plummets and the larger fish, which normally gravitate toward shallow water for feeding advantage, get caught unable to thrive in the low O2 frigid conditions.
Reds can tolerate temps from the low 50s to the upper 80s before they have to relocate, but the "optimal temperatures for spotted seatrout are between 69° - 80°F. They will seek out cooler(deeper) water when it is warmer than 88°. Likewise, when the water is colder, they may hold in deeper channels or holes where the water may be warmer. They may die at temperatures below 48 degrees. (http://recon.sccf.org/sport-fishing/spotted-seatrout)
Another reputable source summarizes as follows: