The migration of juvenile Atlantic salmon
through the lower stretches of coastal rivers,
estuaries and bays to the ocean and is particularly
taxing. First, dramatic physiological changes
take place before smolts are able to transition into
the salty marine environment. Second, they encounter
unfamiliar environmental conditions and a
new complex of piscivorous (fish-eating) predators
including seals, waterbirds and fish as they migrate
downstream. Smolts that successfully navigate
these obstacles must then contend with the challenges
associated with inhabiting a marine environment
as postsmolts and adults.
In the marine environment, Atlantic salmon encounter yet another suite of predators and are also at the mercy of large scale oceanographic processes. For example, climate changes shape the physical, chemical, and biological conditions of the sea which in turn influence marine ecology. Declining return rates of Atlantic salmon worldwide suggest that the conditions Atlantic salmon are exposed to at sea affect marine survival; however, the exact mechanisms by which these conditions impact Atlantic salmon are not well understood.
NEST is particularly interested in learning more
about Atlantic salmon and the conditions they encounter
as they transition into the marine environment.
Globally, this is a much-understudied part of
Atlantic salmon ecology, partially because of the expense
and logistical complications associated with
operating at sea.
In 2001, NEST initiated the Penobscot Bay postsmolt
trawl (PST) survey to sample emigrating Atlantic
salmon. In so doing, NEST researchers hope to
gain a better understanding of the factors specific to
Maine’s Penobscot River and Bay that influence migration,
distribution, and survival.
Although the PST is thus far limited to the Penobscot system, NEST hopes that what is learned will
have broader applicability to Atlantic salmon in general
and sampling programs elsewhere. By improving
its pelagic sampling techniques, NEST will also
be more prepared for an international marine survey program slated to begin in 2008 and aimed at understanding big picture issues such as marine survival.
The Postsmolt Trawl as a Sampling Platform |
The postsmolt trawl survey was conducted from 2001 to 2005 during the smolt emigration period (late spring). A pelagic net was towed behind two vessels (pair-trawled) at the surface and throughout the Greater Penobscot Bay. An aluminum “aquarium” at the net’s codend (pictured right) allowed captured fish to safely swim in a non-turbulent pool of water until the trawl was retrieved. All tows were conducted for 30 minutes except for the offshore tows which were conducted for one hour.
General Sampling: All postsmolts were counted and the following information collected:
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Length (mm) and weight (g)
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fin condition/deformities (can help determine origin)
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degree of smoltification (based on external characteristics)
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marks (e.g. fin clips) and tags (e.g. VIE)
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Scale and genetic samples: Priority for further biological sampling was given to postsmolts with certain tags and/or fin clips, as well as those presumed to be of naturally reared origin. Scale and genetic samples were taken for age and origin (hatchery or naturally reared) determination. |
Gill and blood samples: Gill and blood samples were collected to assess postsmolt physiological condition. Additional gill samples were collected from postsmolts identified by tag as being treated under the ongoing Marical SeaReadyTM project. Data generated from these samples are being used to determine if fish undergoing the treatment demonstrate a physiological advantage over untreated (control) fish. |
Muscle tissue samples: were collected from certain marked fish for a new RNA:DNA study that seeks to provide an alternative way to measure growth from the time of stocking to capture in the PST. |
Stomachs: were extracted from postsmolts that perished due to trawling or sampling (~5% of the total annual catch). Stomach content data is being used to investigate dietary habitats of postsmolts. |
Non-target species: All non-target species captured were identified, counted, measured and weighed. Species that occupy the same space as out-migrating Atlantic salmon provide a unique ecological dataset that improves our understanding of species interactions. |
As the PST allows NEST researchers to sample Atlantic salmon as they migrate to sea, it also provides a very good opportunity to assess the contribution of stocked individuals to the population, as well as the success of different stocking strategies that are varied by timing and location.
In cooperation with Green Lake National Fish Hatchery (GLNFH), NEST carries out an extensive “mark-release-recapture” study on the Penobscot River that extends into the bay. Smolts reared at GLNFH are administered distinguishing marks prior to stocking that allows them to be identified if they are recaptured further downstream in NEST’s rotary screw traps (RSTs) or in the Greater Penobscot Bay during the PST Survey.
In addition, a portion of the parr representing the lower size-range of the smolt population at GLNFH are administered fin clips and stocked in the fall, prior to the spring stocking of their larger counterparts. It is believed that these fall stocked parr will remain in the river and emigrate the second spring after release.
Wild-spawned and hatchery-produced fry (stocked every spring to coincide with the natural emergence of wild-spawned fry from redds). Capturing these unmarked, naturally reared fish enables researchers to evaluate the impact of natural rearing conditions on growth and survival.
What have we learned from the PST?
Since 2001, the PST has captured and sampled approximately 3,400 postsmolts (see map to right for 2005's catches).
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Over 1,000 postsmolts with VIE tags or fin clips have provided valuable information related to different stocking strategies.
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General sampling has allowed NEST researchers to better understand the contribution of hatchery and naturally reared individuals to the postsmolt population.
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Approximately 1,600 scales have been collected and are currently being used to assist NEST researchers in verifying the origin of sampled individuals.
- The 1,000 physiological samples that have been collected since the PST's inception are currently being analyzed and will help inform us about how well smolts are transitioning from freshwater to estuaries to the marine environment.
We've learned what smolts are eating...
Click for Enlarged Graph
The contents of the stomachs extracted from Atlantic salmon postsmolts captured from 2001-2005 reveal that Atlantic herring and krill are the major prey items of Atlantic salmon in Penobscot Bay. Unidentified fish remains are most likely those of Atlantic herring, whereas miscellaneous fish include wrymouth, rock gunnel, radiated shanny, searaven and sculpin species. Miscellaneous crustaceans include amphipods, copepods and isopods.
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And about which fish species have the opportunity to interact with smolts as they leave Maine's rivers...
Non-target species composition (by weight) of PST tows from 2003 – 2005 reveals that Atlantic herring are the dominant species in the Upper and Middle Bay Areas, and lumpfish are the dominant fish species in the the offshore Outer Bay
Areas. Other species include American shad, striped bass,
sea lamprey, rainbow smelt, Atlantic tomcod, capelin, Atlantic
silversides, threespined stickleback, hake species, and
butterfish.. |
Data collected during the PST are used to guide future sampling efforts and to help scientists and managers better understand the factors influencing Penobscot River Atlantic salmon dynamics.
Although there is no simple answer to why adult Atlantic salmon return rates continue to decline, PST data may help to contribute to a better understanding of postsmolt ecology and current restoration stocking strategies. The PST continues to be part of a larger, ongoing NEST endeavor to improve understanding related to Atlantic salmon, especially during the critical time at which they transition from freshwater into marine environments. |
With camouflage markings along its sides, the parr life stage of an Atlantic salmon (above) is adapted for river life. After spending two years in freshwater, parr transform into smolts (below) and become adapted for life at sea. During this transformation, smolts become physiologically capable of excreting salt and also take on a more silvery, streamlined form typical of marine pelagic fishes. From the time they leave their natal river as smolts to when they return as adults to spawn, Atlantic salmon face numerous challenges, many of which are not well understood.
Migration routes of Maine's Atlantic salmon
The aquarium is hoisted on board the trawling vessel after a 30 minute tow.
Stomach samples tell us that postsmolts feed on a variety of prey items, including krill (top) and juvenile herrings (bottom).
Above, a hatchery reared smolt from GLNFH is marked with a Visual Implant Elastomer tag (VIE). VIE tags are specific to stocking date and site.
Location of tows and number of postsmolts
caught during the PST in 2005
Click for enlarged map
And about the contributions of stocked individuals...
Click for enlarged graph
Above graph: Analysis of origin-related data (e.g. observed hatchery-administered marks, scales, fish length/weight, and dorsal fin condition) from the 2003-2005 PST reveals that the overwhelming majority of the postsmolts caught during the trawl were stocked as smolts.
Click for PDF
Above graph: A length-frequency distribution of the postsmolts caught during the 2005 PST reveals a bimodal distribution. Fall stocked parr tend to occur in the lower mode (120-160 mm) and individuals stocked as smolts tend to dominate the upper mode (160-220 mm). The presence of a bi-modal length distribution may afford NEST another means by which to identify unknown origin fish (i.e. individuals falling within the lower mode as stocked parr and those in the upper mode as stocked smolts). These results suggest that parr stocked individuals may make up a larger proportion of the total catch than previously thought. Studies are currently underway to explore this issue further.
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