Surrounding the volcano is the Arenal Conservation Area, a lush forest reserve, home to over 3,459 species of animal! Adjacent to the volcano you will find the man-made Arenal Lake, which serves as the reservoir for a hydroelectric plant that supplies a large part of the country’s electric energy. The area provides an opportunity to visit nature, or take a canopy tour, a horseback ride, kayaking and of course, for fishing.

According to William Bussing, an ichthyologist and professor of biology at the University of Costa Rica, 127 species of freshwater fish of 33 different families can be found in the inland waters of Costa Rica.

Located at the base of a highly active volcano, surrounded by a tropical jungle rich in wildlife, this beautiful lake is over 15 miles long.   Lake Arenal is famous for its rainbow bass, (PARACHROMIS DOVII  CICHLIDAE),   a member of the cichlid family that is fished just as you would large mouth bass.   Gleaming in iridescent turquoise, with delicate shadings of pink and purple and feathered ends to its caudal and dorsal fins and a distinctive hump on the head of the mature male; the rainbow bass or guapote frequently run five to eight pounds, with an 11 ½ pounder holding the world record. 

Rainbow bass or Guapote 
Has been described as: A beautiful fish, very strong, a good fighter, and delicious to eat.
To provide you with more information, let us send you a comprehensive article published by Jerry Ruhlow  (Costa Rica Outdoors Magazine):

“Lake Arenal: Mecca for Inland Anglers

A scenic three hour drive from San José, Lake Arenal is the premier fishing lake in Costa Rica, fed by a multitude of rivers and loaded with rainbow bass.

Originally little more than a swampy lagoon, the lake more than doubled in size after construction of a dam was completed about 1978, and at high water is now more than 20 miles long, surrounded by verdant forest. Arenal Volcano looms majestically at the southern end, and recent satellite photos indicate the presence of archaeological ruins in the fertile valley that is now the lake bed that date back 2,000 years. Winds that average 25 miles per hour—and sometimes whip up to twice that velocity—blow out of the north from November through April, earning the northern end of Lake Arenal a reputation as one of the world's top wind surfing locations.

But the many small coves and points on the more protected lower end of the lake make it possible to fish Arenal year around. The bite depends more on water level and angler's skill than it does the wind.
What the fishermen are hunting is rainbow bass (Cichlasoma dovii) which earned its name because of the subtle gradations of color and the fact that they are fished just as you would for largemouth bass.
A member of the cichild family it is related to the peacock bass (Cichla temensis) found in the lakes and rivers of South America and known locally as guapote ("the most handsome" in Spanish). It is shaded in pastel hues, has a fearsome set of teeth, and feathered dorsal and caudal fins.

Contrary to some reports, the rainbow bass was not stocked in the lake, but moved in from the many feeder rivers and took up residence naturally, as did machaca and mojarra. Escapees from a breeding program added a few tilapia to the population and there are rumors of a surreptitious planting of largemouth bass but no confirmed reports of any every being caught.

Record for rainbow bass is 12 lb. 9 oz., caught in Costa Rica's Lago Hule, and listed as a record by the IGFA—for reasons of its own—as guapote. It was caught in 1991, but only recently submitted and approved.
There are a few hotels and lodges at the lake that offer accommodations, boats and guides.
Costa Rica Outdoors interviewed Bob Hohl, Doug Kralick and John Taylor, three of the country's top rainbow bass fishermen and consistent winners in the local tournaments as to when, where and how to fish the lake.
Consensus is that fishing is best when the water level in the lake begins to recede from its peak level.

Hohl first fished the lake in 1978 soon after it opened, and has been a regular ever since. Largest he's caught have been in the 10-pound range.
"I keep a journal, and in looking back over recent years I've caught fish every month, but I can't say which is best," he said. "It used to be that the lake was at its highest level by January and lowest in September, but now they release water intermittently for hydroelectric power and irrigation, so you never know."

Hohl also said he likes to fish the lake around a full moon, while Taylor feels the barometric pressure is important and gets best results when the barometer is up, just before a front moves in. "August, September and October have been my best months, Taylor said."It's usually raining to beat hell but the wind lies down." Kralick said he doesn't watch the barometer, but feels it has a lot to do with it. "I like to fish in a light rains which seems to keep the fish more active during the day," he added. "When the water is high and starting to drop it pulls the fish tighter into the structure, but every time you go it's a different situation."

He's been fishing the lake since 1989, and his largest to date was a 10 1/2 pounder taken in April, 1995.
Taylor started fishing Arenal in 1983, has nailed a lot of six to seven pounders, with his largest 8 1/2 pounds.
The three experts believe vehemently that the big fish should be released and keep only the smaller fish for immediate consumption, which ain't a bad idea considering the unanimous vote of anyone who's ever set fork to a rainbow bass that's it's the best eating of any fish in the world.
All three were in accord that you have to cover a lot of water, and believe in working the sticks and brush with spinner baits in the early morning and late evening, then going deeper with jigs and crank baits when the bite drops off, jumping from point to point and looking for structure.

If you pull some fish off a particular spot, mark it well and go back for another try later in the day.
A tournament bass fisherman in the US, Kralick is also in agreement with Hohl and Taylor that while they are fished alike, the guapote is a far more voracious predator than black bass and put a far better fight.
Unlike the black bass, however, the rainbow bass is a deep fighter, staying deep and rarely breaking the surface. They seldom come back if they miss the lure on the first strike and invariably attack a bait from behind rather than coming on the head as does a largemouth. For that reason Doug recommends trimming the trailers to bring them closer to the hook on spinner baits.

"Guapote are a much harder fighting fish than black bass, they have bigger fins around their bite which creates more torque, and if they rap the line around a tooth can easily break 30-pound line," he adds.
Lures should be worked exceptionally slow as the rainbow bass is slower and more deliberate in his attack than those bass at home.

If you're packing a tackle box for Arenal, they recommend lots of spinner baits, jigs and a few crankbaits, bemoaning the fact that the one-ounce Rattletrap is no longer manufactured. It is a favorite for casting or jigging deep. Options are B ombers, medium diver Rapalas and similar lures that will get down 12- to 15-feet. Buzz baits and surface poppers sometimes produce in the early a.m., but are not as effective as spinner baits.
White, perhaps with a bit of chartreuse or yellow trim, are the overwhelming choice for skirts, fished with crawdad trailers.
Most serious anglers have switched from 20 pound mono to 50 pound spiderwire, as most of the fishing is done in heavy structure and lures are a mite pricey in Costa Rica.
It is also recommended that you change the factory hooks on crank baits to something larger or heavier.
Medium and medium heavy rods are favored, Taylor preferring a shorter 5 1/2 foot Boron stick, with Kralick recommending a 7 to 7 1/2 foot heavy action graphite rod for jigs; a 6 1/2 to 7 foot softer graphite medium action for crank baits. Hohl opts for his 6 1/2 to 7 foot Phenix rods, medium to heavy action”.

Most experts recommend:
To fish Rainbow Bass, Machacas and Mojarras at Lake Arenal