Nekkton and Crabs
Terms
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- Nekton
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-actively swimming pelagic organisms
-mostly fishes, but can also include squid, scallops, crabs, shrimp, whales. - Typically high biomass levels
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-important commercial fisheries
-seasonal in estuaries
-H.L. Menken described the Chesapeake as a great "protein factory"
-Because of economic value they are often intensively investigated - Morphology
- eg. blue crab life history and aquaculture
- Physiology
- eg. disease resistance in striped bass
- Ethology
- crab aggression is a major factor
- Taxonomic considerations
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-mostly fish but includes diverse invertebrates and even mammals
-in estuaries 8-15 species are 90% of the total biomass
-marine and fresh water systems have higher diversity - Anguillidae
- freshwater eels
- Clupeidae
- herring-like fish
- engraulidae
- anchovies
- Sciaenidae
- sea trout, drum, croakers
- seranidae
- bass
- Decapod crustacea
- most widespread 5 pairs of appendages
- The blue crap, Callinectes sapidus
- is the most important chesapeake crustacean
- "The beautiful swimmers"
- William Warner, won pulitzer prize
- Crustacea
- The most important invertebrate nekton
- Sapidus
- "good to eat"
- Ecological or Functional Classification of Nekton
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-Shallow water
-Pelagic
-Epibenthic - Shallow water fishes
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-Estuarine edges and "edge effect"
-Marshes
-Grassbeds
-Tidepools
-Generally small
-No migratory behavior
-Schools of killfish and silversides are common in the bay - Pelagic Species
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-Swim freely in water column
-Usually in schools near surface...blue fish feeding create turbulence and an oil slick
-Usually strong migratory behavior anadromous
1. Plankton feeders-Shad
2. Carnivores-blue stripers - Plankton feeders
- May specialize in phytoplankton or detritus
- Herring--
- Alosa "alosids"
- Anchovies
- Anchoa
- Menhaden
- Brevoortia
- Herring and Menhaden
- modified gills to filter zooplankton and phytoplankton
- Carnivorous pelagic fish
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-Feed on pelagic fishes, forage fish, like menhaden or herring
-bluefish Pomatomus Saltatrix
-Striped bass Monrone Saxatilis - Epibenthic or demersal
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-Bottom oriented species. Often swim near bottom
-Most diverse group due to
1. Substrate variety
2. Biotic and abiotic interaction including repro. strategies, migration patters, food availability
-Catfish, flatfish, croakers, hogchoker. - Adaptations to nektonic and estuarine existence
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-swimming organs
-steamlined body
-specific gravity near water
-ability to extract oxygen rapidly - nektonic adaptations
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-shad have large eyes to escape predation
-anadromous fish have keen olfactory systems
-locate natal rivers by smell
-natural history of the senses.. diane ackerman - Stressful estuarine life
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-fluctuating chemical and physical environment
-places energy demands on fish...salinity and osmoregulation
-many species can't adapt so the number of estuarine species is low
-however, many species spend part of their life history in an estuary.. - blue crab
- callinected sapidus
- blue crabs information
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-key component of Bay culture, economy, and cuisine
-native americans consumed crabs (midden data) - Crab life history
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-Mating occurs June-October and peaks in July-August
-male joins female at pubertal molt...deposits sperm packets..."doublers"
-postcopulatory embrace for several days while female shell hardens
-sook run...fertilized females migrate to south of Bay
-females release egg mass
-eggs hatch producing planktonic zoea which drift out over continental shelf
-megalops enters and move up Bay through tidal action and salt wedge
-45 days after hatching juvenile crabs develop
-immature crabs will molt up to 20 times during their lives
-mature in 12-16 months, probably only live in 3 years
-females migrate to the souther bay in october. - soft crabs
- value added product
- Controlling molting
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-eyestalk glands produce an inhibitor to molting
-remove eyestalks and remove inhibitor
-molt hormone is ecdysone
-soft crabs on demand - Crab shells
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-made of an important biopolymer called chitin
-resistent to biological degradation
-can be induced to have a positive charge.. can be used in chemical reactions - factors contributing to blue crab decline
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-fishing pressure... 90% of crabs spawning potential removed each year.. 70% may be acceptable
-water quality...
1. nutrients, algae and anoxia
2. decline of SAV
3. increased stress=increased disease
-scientists believe that reduced population levels make the blue crab more susceptible to other disturbances
-many argue that the blue crab has been exploited to near collapse - management needs for crabs
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-reduced length of crabbing season
-ban winter dredging
-limit days or hours of work.
-sanctuaries
-limit commercia or recreational licenses
-limits - reduce wasteful crab harvesting
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-10-30% of crab pots lost each year
-crabs enter, die and attract other crabs... self baiting.
1. biodegradable escape hatches
2. Ids on pots for accountability
3. cull rings allow small crabs to escape
-ban harvest of sponge crabs
-Ernst calls for a coordinated management between MD and VA... winter dredging plan