NMFS Northeast Region

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Cherryfield, Maine – When a team of biologists pulled a young salmon from a trap at Little Falls on the Narraguagus River April 20, they were looking at one of the first migrating smolts of the 2001 season. The biologists weighed and measured the two-year-old fish, took small tissue and scale samples, and released the smolt less than 60 seconds later on the down-river side of the trap.

Over the next month, biologists plan to trap and release several hundred more smolts in a study that is providing important information about the endangered Atlantic salmon populations in Maine. Smolt monitoring is currently being conducted on four Maine rivers by scientists from NOAA's National Marine Fisheries Service (an agency of the Department of Commerce) and Maine's Atlantic Salmon Commission (ASC).

Biologists estimate that thousands of endangered Atlantic salmon will leave seven Maine rivers and one brook over the next seven weeks. The fish that survive this phase of their migration will spend two years at sea before returning to their home rivers to spawn in 2003.

In recent years, very few spawners have returned to the seven Maine rivers – it is estimated that only 75 to 110 salmon returned in 2000, down from nearly 300 fish as recently as 1991. Salmon experts are trying to identify the factors causing the decline and trying to determine whether the most significant losses are occurring during the freshwater or salt water phase of the fish's life.

"The smolt trapping project is providing important new information about how many young fish are produced in these rivers," said Tim Sheehan, a NOAA Fisheries biologist who tends the smolt traps. "We're learning more about where mortalities occur as the fish leave fresh water and move out into the ocean."

Atlantic Salmon Commission biologist Lonna Perry nets fish from the holding box of a rotary screw fish trap. Two-year-old smolts like the one below are measured, marked, and released into the river to continue their journey to the ocean.

Before the smolt trapping project began, scientists had no way of knowing how many salmon were leaving the rivers every spring. To estimate the number of fish lost during the marine phase, they could only compare comparing the number of juvenile fish in a river the autumn before migration with the number of adult fish that returned from the ocean 1-3 years later. In this method, the number of fish estimated to have been lost in the marine environment included an unknown number of juveniles that did not survive their final freshwater winter, plus any migrating smolts that did not survive the difficult transition from fresh to salt water.

In 1996, John Kocik of NOAA Fisheries and Ken Beland of the Atlantic Salmon Commission started smolt trapping studies to provide better estimates of the number of smolts that made the journey to sea. The project began on the Narraguagus River, with a rotary screw trap – a passive, water-powered fish capture system that had never before been used in New England. Smolts caught in the trap are weighed and measured, and small tissues samples and/or scale samples are sometimes taken, and then the fish are released to continue their migration to the ocean. In related studies, some of the smolts receive surgically implanted ultrasonic tags that provide information about how those fish fare when they encountered salt water farther downstream.

The data collected over the first five years of study have provided scientists with a new view of Atlantic salmon ecology. For the first time, scientists can estimate how many juvenile fish died in the winter before their downstream migration.

"We are finding that a substantial number of smolts are lost in the fresh water environment," Sheehan said. "It appears that conditions in the river play a significant role in determining how many fish eventually return to the river to spawn."

In addition, when smolt-trapping data are combined with data from ultrasonic tagging projects, scientists can measure the loss of salmon smolts in estuaries and nearshore environments. These data will help separate mortality in the first month at sea from the rest of the 1-3 year migration.

"The information from this research should help focus the salmon managements effort more directly on those problems that have contributed most to the decline in recent years," Sheehan said.

The smolt trapping study has been successful enough that NOAA Fisheries and the ASC have extended the range of the study both north and south of the Narraguagus. In 1999, NOAA Fisheries, in cooperation with ASC, began monitoring the smolt migration on the Pleasant River. The 2001 season marks the beginning of smolt trapping on the Dennys (ASC) and the Sheepscot (NOAA Fisheries).