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Population dynamics

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Fish population dynamics (image credit: Katherine Nicholson, slideplayer.com)

Fish population dynamics is the study of change in a fish population over time.  Fundamentally, a population size after some time interval equates to the population size before that interval plus births (i.e., recruitment) and immigration and minus mortality and emigration.  Many fish populations follow a logistic pattern of density-dependent growth.  Beginning with a population size where space and food are not limiting, the population grows rapidly in an exponential pattern; at a certain population density, population growth slows and stabilizes at a given carrying capacity.

Density-dependent factors governing fish population dynamics include competition, predation, disease, and parasitism.  Fish population dynamics can also be driven by density-independent abiotic factors such as temperature, dissolved oxygen, and water chemistry.

In fisheries, these basic population dynamics principles can be applied to harvest strategies to estimate maximum sustainable yield (MSY) with the goal to prevent overfishing.

Phenology

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Phenological events triggers Sockeye Salmon migrations (photo credit: USFWS).

Phenology is the study of seasonal or periodic cycles in ecosystems.  It is, essentially, “nature’s calendar.” For fish, phenology is often linked to timing of important life events such as spawning, migrations, and hatching.  These events are triggered by non-biological factors like day length, temperature, and precipitation.  If the timing of the main “cue” shifts, the alteration can have significant implications for the fish populations that have evolved to optimize the timing of their life events with that factor.

Many fall–spawning Pacific Salmon populations (Oncorhynchus spp.), for example, are beginning their freshwater migrations earlier than in the past (e.g. Kovach et al. 2015).  This consistent trend across species and populations strongly suggests that a shared environmental driver (i.e., climate change) is responsible.  Unfortunately, these altered behaviors can be maladaptive – Cooke et al. (2004) found that increased pre-spawn mortality rates – sometimes in excess of 90% – for several stocks of Fraser River sockeye salmon (Oncorhynchus nerka).

 

References

Cooke, S.J., Hinch, S.G., Farrell, A.P., F., L.M., Jones, S.R.M., Macdonald, J.S., Patterson, D.A., Healey, M.C., Van Der Kraak, G., 2004. Abnormal Migration Timing and High en route Mortality of Sockeye Salmon in the Fraser River, British Columbia. Fisheries 29, 22–33. doi:10.1577/1548-8446(2004)29

Kovach, R.P., Ellison, S.C., Pyare, S., Tallmon, D.A., 2015. Temporal patterns in adult salmon migration timing across southeast Alaska. Glob. Chang. Biol. 21, 1821–1833. doi:10.1111/gcb.12829

Panmictic

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Panmictic refers to a random mating strategy, frequently employed by fish, where breeding is just as likely to occur between any two individuals in a population as between any two others.  Mating in this way is not influenced by any any environmental (e.g., geographic proximity), hereditary (e.g., timing of spawning), or social interaction (e.g., polygamous mating systems).

American Eels have a panmictic population.

In a population genetics context, if a species is a panmictic population, there is no genetic evidence of population structure throughout its range.  For example, American Eels (Anguilla rostrata) are catadromous fish found from the Caribbean to Greenland.  However, they all migrate to breed in the same location in the Sargasso Sea so genetic samples from throughout their North American distribution range show a complete lack of genetic differentiation, or complete panmixia.

 

Placoid denticles

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While these Great White placoid denticles may look like scales, they are actually modified teeth (Trevor Sewell/Electron Microscope Unit, University of Cape Town).

While these Great White placoid denticles may look like scales, they are actually modified teeth (Trevor Sewell/Electron Microscope Unit, University of Cape Town).

Placoid denticles are found on sharks, rays, and chimaeras.  Not really a true “scale,” like ctenoid or cycloid scales, placoid denticles are actually modified teeth.  They have an inner tissue component, which contains both blood vessels and nerves, that is covered by a layer of dentin and an outer enamel.  They form a tough protective skin layer for sharks, rays, and chimaeras and also have shown to reduce friction and drag so that these fish can swim more efficiently through water.

Photophore

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Photophores from a Splitfih Flashlight fish can be seen from 100 ft away!

Photophores from a Splitfin Flashlight Fish can be seen from 100 ft away!

Photophores are organs that are used by fish (and invertebrates) to produce light either by chemical reaction or through symbiotic bacteria capable of bioluminescence.  Most fish that use photophores live in the deep sea where light from the surface is limited.  Like a firefly in the sea, some of these fish use photophores to attract mates; others use photophores as counterillumination  and camouflage; others use their photophores like search lights to find prey or avoid predators; and still others use photophores for multiple purposes.  Splitfin Flashlight Fish (Anomalops katoptron), for example, use their photophores to communicate with other flashlight fish, attract prey, and confuse predators.  They are believed to produce the brightest bioluminescence of any organism – their light can be seen from over 100 feet away!

For other examples of fish with photophores, check out the “twinkle twinkle little fish” post on the Fisheries Blog!

Poikilotherm

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Poikilothermic fish have no control over their body temperature and their core body temperature can fluctuate broadly.  While some ectothermic stenotherms thermoregulate their body temperature by inhabiting constant temperature environments, internal temperature of poikilotherms can widely vary.

Thoughout their lives, Steelhead's internal temperature varies considerably (NPS).

Thoughout their lives, Steelhead’s internal temperature varies considerably (NPS).

 

Potamodromous

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A potamodromous fish, like an anadromous or catadromous fish, is a migratory fish.  Unlike anadromous or catadromous fish, a potamodromous fish spends its whole life in fresh water.  Generally, these migrations are for spawning purposes and cover short distances: from an upstream tributary to a mainsteam river or between connected lake and river systems.  Some species, for example the endangered Colorado Pikeminnow (Ptychocheilus lucius) have very extensive fresh water migration routes – reportedly up to 300km.  Like salmon, the pikeminnow uses homing to reach very specific spawning locations with a high degree of fidelity.

The endangered Colorado Pikeminnow migrates long distances only in fresh water.

The endangered Colorado Pikeminnow migrates long distances only in fresh water.

For more information, check out the “CAN YOU SAY ANADROMOUS, CATADROMOUS, AMPHIDROMOUS, OCEANODROMOUS, OR POTAMODROMOUS?” post on The Fisheries Blog!

Pelvic fins

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The pelvic fins are paired fins found on the ventral (bottom) side of fish.  In teleost (bony) fishes, pelvic fins placement gives some indication of evolution.  For more basal groups, the pelvic fins are located at mid-body in the abdominal region.  For more derived fishes, the pelvic fins are located in a thoracic or even jugular (throat) position.

Male guppies have specialized pelvic fins

Male guppies have specialized pelvic fins

Some fish, such as Guppies (Poecilia reticulate) exhibit sexually dimorphic pelvic fins, where the males and females do not have the same fins.  The pelvic fin’s first and second rays are significantly shorter on the male Guppy than the Female guppy.  Research suggests that this specialization may assist with reproduction.

Other fish, like the Devil’s Hole Pupfish (Cyprinodon diabolis) lack pelvic fins entirely.  At less than one inch long, the Devil’s Hole Pupfish is the smallest of the desert pupfish species and one of the world’s rarest species (only 65 fish were counted in a Fall 2013 survey).  It is only found in one location on earth, Devil’s Hole, Nevada, and has evolved over thousands of years to survive in its harsh 93oC, low oxygen waters.

Devil's Hole Pupfish lack pelvic fins

Devil’s Hole Pupfish lack pelvic fins

Pectoral fins

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Pectoral fins are on the sides of the body

Pectoral fins are on the sides of the body

The pectoral fins are the two fins located on the sides of a fish (or marine mammal).  These fins are primarily responsible for control of directional movement, up and down or side to side.  Pectoral fins can come in all shapes and sizes which fill different functions for different fish.  The pectoral fins of a Coelacanth (Latimeria chalumnae) have a wide range of motion and are capable of “sculling” like the oar of a boat.  These specialized pectoral fins are useful for making small correction movements to maintain a Coelacanth’s position, hovering just off the ocean floor.  The pectoral fins for Bluefin Tuna (Thunnus thynnus) are retractable; they fit into slots so that when they are retracted, they are flush with the side of the fish.  For a fast-moving fish in open oceans where it doesn’t often have to change direction quickly, this feature is highly efficient – it reduces drag and saves energy.  Some flatfish, like the Hogchoker (Achirus fasciatus), lose their pectoral fins all together.  Their highly derived body shape and life history eliminates the traditional role for pectoral fins.

Coelacanths scull with their pectoral fins

Coelacanths scull with their pectoral fins

Pelagic zone

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The pelagic zone is divided into layers based on light

The pelagic zone is divided into layers based on light

The pelagic zone is the region of a body of water (lake, river, or ocean) that is not associated with the bottom (see benthic zone) or shore (see littoral zone).  This habitat zone is truly a three dimensional habitat space.  Some fish that occupy the pelagic zone never encounter the bottom or shore throughout their lives.  Because the pelagic zone is a nutrient poor environment, large fish have two basic strategies to get meals – either swim long distances in search of nutrient-rich prey (like many oceanic sharks and tunas) or drift with currents and eat nutrient-poor prey (like the Ocean Sunfish Mola mola).  The pelagic zone is divided into zones based on light penetration from the surface:

  • Epipelagic (sunlight; 0-200m)
  • Mesopelagic (twilight; 200-1,000m)
  • Bathypelagic (no light; 1,000-4,000m)
  • Abyssopelagic (4,000m – ocean floor)
  • Hadopelagic (deep sea trenches)