Basque cod with sous-vide

Per this paper, you want an extraction time of 15-60 mins at 113F / 45C. This patent uses temperatures below 131F / 55C. Both of those temperatures may be too low to make the cod actually taste good / cooked. 140F is a fairly typical temperature for sous vide fish. While 113F has some interesting results, it may or may not work for your dish as the result comes out quite different than standard cooked fish. However, I don't think that using a higher heat will have a negative effect on the gelatin extraction, so I'd start with 140F and work down from there.

This process may cause other problems though. In addition to extracting the gelatin, any other juices from the fish will mix with your gelatin. In a pan, these may steam off, but with sous vide you're stuck with them. I think you're likely to end up with a mixture that's much more watery than if you used a pan. You may find that this needs to be boiled down a little bit in order to get the desired consistency of your emulsion.

As always with sous vide, you've also got potential safety issues. You'll never reach pasteurization at 113F. You should check Douglas Baldwin's A Practical Guide To Sous Vide for cooking times (which are highly dependent on width).

Salmon mi-cuit is cooked as low as 108F per Douglas Baldwin but this is a "fast" cooking technique that requires only 15-20 min cooking max which is not long enough to render gelatin.

You can render gelatin safely from skin and bones (or whole fish) sous vide for longer periods of time at any temperature at or above 131F.

I would consider rendering the gelatin separately for a long period of time, reserving it, and putting it back with fish possibly done at a lower temperature (depending on what level of doneness you want).

FWIW, the Douglas Baldwin section on Salmon ‘Mi-Cuit’ has a number of interesting tips on low temperature sous vide for fish.

Edited excerpts from: http://www.douglasbaldwin.com/sous-vide.html#YTSalmon

Salmon ‘Mi-Cuit’ - Douglas Baldwin

While salmon mi-cuit is a popular among sous vide enthusiast, it should never be served to immune compromised individuals. The low cooking temperatures in this recipe are not sufficient to reduce the number of foodborne pathogens or parasites. Since the prevalence of the parasite Anisakids simplex may exceed 75% in various types of fresh U.S. commercial wild salmon (National Advisory Committee on Microbiological Criteria for Food, 2008), I recommend either freezing the fish (below –4°F/–20°C for at least 24 hours) to kill the parasites or pasteurizing the fish using the times and temperatures in Table 3.1.

. . . . . .

Set the temperature of the water bath to 108°F (42°C) for rare salmon, 122°F (50°C) for medium--rare salmon, or 140°F (60°C) for medium salmon. Then prepare a 10% salt water solution (100 grams salt per 1 liter cold water).

. . . . . .

Cut the salmon into individual servings and vacuum seal. For rare and medium-rare salmon, cook the salmon for 15–20 minutes. For medium salmon, pasteurize it for the time listed in Table 3.1. Then remove the salmon from its pouch, garnish with crisped salmon skin, and serve immediately.