Stable Ocean Hypothesis
Encyclopedia
The Stable Ocean Hypothesis (SOH) is one of several hypotheses within larval fish ecology
that attempt to explain recruitment variability (Figure 1; Table 1). The SOH is the notion that favorable and somewhat stable physical and biological ocean
conditions, such as the flow of current
s and food
availability, are important to the survival of young fish larvae and their future recruitment. In the presence of stable ocean conditions, concentrations of prey form in stratified ocean layers; more specifically, stable ocean conditions refer to “calm periods in upwelling
ecosystem
s (sometimes called 'Lasker events')” that cause the water column to become vertically stratified. The concept is that these strata concentrate both fish larvae and plankton
, which results an increase of the fish larvae feeding because of the density-dependent increase in predator-prey interactions. Lasker
is attributed with constructing this hypothesis in the late 1970s by building on previous larval fish research and conducting his own experiments. He based the SOH on case studies
of clupeid
population fluctuations and larval experimentation.
cited the disconnect between spawning stock biomass
and the recruitment of numerous fish species. One explanation of this disconnect suggests that larval recruitment is influenced by spatial and temporal patterns of their food, like phytoplankton
or zooplankton
, which can be greatly impacted by ocean currents and mixing. In his publication Marine fish larvae: Morphology, ecology, and relation to fisheries (1981), he points out, for example, the Peru
vian anchovy
fishery collapse that resulted from a dramatic decrease in population size during the early 1970s. Officials and researchers from the Peruvian government and United Nations Food and Agriculture Organization submitted that the causal factors were a combination of strong fishing pressure and weak year classes that resulted in insufficient reproduction and recruitment to support the fishery
. This explanation seemed to explain the diminished population trends of similar species from other regions, including the Pacific and Japanese sardine
s and the Atlanto-Scandian herring
.
Lasker, however, opposed this conclusion while citing the seemingly miraculous recovery of the troubled Japanese sardine population from scarcity (e.g. thousands of landed tons) to prominent abundance (e.g. more than a million landed tons). Another researcher studying the rebound of the Japanese sardine, Kondo (1980), identified an unusually strong 1972-year class, which produced successful recruitments in the years that followed. Kondo also noted altered ocean current patterns that increased zooplankton availability in spatio-temporal coincidence with the hatching of the sardine larvae. The result was increased larval survival and the eventual rebound of the population. Thus, the observed trend is that strong year class anomalies can have major impacts on population sizes and their future stability and growth. This concept also illustrates how plankton abundance and ocean currents can be driving factors associated with such trends. Clearly, these patterns become important when considering the predictive models necessary to manage and sustain important fisheries and the stocks that support them.
He observed that anchovy larvae would first feed only when introduced to layers from below the surface containing high levels of chlorophyll
and certain species of phytoplankton. These feeding-suitable phytoplankton assemblages were correlated with non-mixing thermocline
layers. This conjecture was supported when foul weather swept up during the study and mixed the previously stratified layers (including a particulate layer below the surface) that had been sampled and introduced to the larvae. Samples derived from the same depths that proved ideal to induce first feeding prior to the storm no longer contained the necessary phytoplankton varieties and abundances; as a result, the young larvae did not feed on the post-storm samples and were unable to survive. Thus, Lasker eventually hypothesized that high-energy events that cause de-stratification of ocean layers have negative impacts on first-feeding northern anchovy larvae by destabilizing and decreasing their prey availability.
associated with the SOH (Blaxter and Southward, 1992). Three major limiting conditions include, but are not limited to:
1) Species and region specific:
SOH is based primarily on experimental studies of Pacific anchovy in the California Current System
(CCS) and should, therefore, be used cautiously when extrapolating to other species and regions (Heath, 1992).
2) Life stage specific:
Lasker’s suggestion that storms ultimately have negative impacts on first-feeding anchovy larvae should come with the disclaimer that this applies only to first- feeding larvae entering the critical period. Older anchovy larvae can, in fact, be positively affected by storm
activity, which often prompts increases in planktonic production (Heath, 1992).
3) Does not consider storm-absent survival of larvae:
Admittedly, one study supported the SOH by documenting a correlation between increased daily mortality rate
s of first-feeding anchovy larvae and intervals of unperturbed, storm-absent conditions. Yet it is important to realize, as Peterman and Bradford did, that all this evidence does not affirm a certain relationship between larval survival and severe weather
; this is particularly true given the lack of data regarding larval survival in the absence of storm events for context and comparison (Heath, 1992). Furthermore, Peterman and Bradford (1987) did not find a strong relationship between the larval survival rates documented adjacent to wind/storm events and recruitment levels observed months afterward.
Article written by Anne Dowling
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
that attempt to explain recruitment variability (Figure 1; Table 1). The SOH is the notion that favorable and somewhat stable physical and biological ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
conditions, such as the flow of current
Current
- Science and Mathematics :* Electric current* Current , including** Ocean currents** Air currents** Current - currents in rivers and streams* Current density, mathematical concept unifying electric current, fluid current, and others...
s and food
Food
Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...
availability, are important to the survival of young fish larvae and their future recruitment. In the presence of stable ocean conditions, concentrations of prey form in stratified ocean layers; more specifically, stable ocean conditions refer to “calm periods in upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s (sometimes called 'Lasker events')” that cause the water column to become vertically stratified. The concept is that these strata concentrate both fish larvae and plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
, which results an increase of the fish larvae feeding because of the density-dependent increase in predator-prey interactions. Lasker
Reuben lasker
Reuben Lasker was a fisheries scientist known for his contributions to larval ecology, particularly the Stable Ocean Hypothesis.-Early life and education:...
is attributed with constructing this hypothesis in the late 1970s by building on previous larval fish research and conducting his own experiments. He based the SOH on case studies
Case study
A case study is an intensive analysis of an individual unit stressing developmental factors in relation to context. The case study is common in social sciences and life sciences. Case studies may be descriptive or explanatory. The latter type is used to explore causation in order to find...
of clupeid
Clupeidae
Clupeidae is the family of the herrings, shads, sardines, hilsa and menhadens. It includes many of the most important food fishes in the world.-Description and biology:...
population fluctuations and larval experimentation.
| Author | Year | Name | Summary | Comments |
| Hjort | 1914 | Critical Period Hypothesis | *Larval survival pivots on whether or not a larval fish successfully feeds during the “critical period” when it transitions from relying on its yolk sac Yolk sac The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals... to exogenous food sources |
*Myopic view of larval starvation Starvation Starvation is a severe deficiency in caloric energy, nutrient and vitamin intake. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, death... being the primary factor in larval recruitment variability |
| *Cites marine fisheries of northern Europe | ||||
| 1926 | Aberrant Drift Hypothesis | *Larval recruitment is affected by wind Wind Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space... s and ocean current Ocean current An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun... s which cause dispersal of eggs Egg (biology) An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing... and larva Larva A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle... e, removing them from essential larval and juvenile habitat |
*Provided the foundation for future larval recruitment paradigms, but was under-tested until recently, when both starvation and oceanographic movements have been recognized as important factors | |
| *Also cites marine food fisheries of northern Europe, like cod | ||||
| Cushing | 1974; 1990 | Match-Mismatch Hypothesis | *Success of larval recruitment is linked to a temporal alignment of fish reproducing, larvae hatching, and plankton (prey) blooming (generally associated with spring) | *An evolved hypothesis combining both of Hjort’s founding concepts |
| *Cites Atlantic cod | ||||
| Lasker | 1978 | Stable Ocean Hypothesis | *When tranquil ocean conditions occur in upwelling systems, the water column becomes stratified in layers that concentrate larvae and plankton, which increases successful larval feeding, which increases larval recruitment and contributes to year class strength | *Relevant to larval survival, but does not correlate strongly to variations in recruitment (Peterman and Bradford 1987) |
| *Cites northern anchovies in the Pacific | ||||
| Cury and Roy et al. | 1989; 1992 | Optimal Environmental Window | *In upwelling systems, wind, storm, and other energetic events cause turbulence that, within an optimal range, increase larval recruitment; dependent upon the presence/absences of Ekman transport Ekman transport Ekman transport, part of Ekman motion theory first investigated in 1902 by Vagn Walfrid Ekman , is the term given for the 90 degree net transport of the surface layer due to wind forcings... |
*Builds upon Lasker’s Stable Ocean hypothesis |
| Hypothesis | *Cites Peruvian anchoveta, Pacific sardines, and West African sardines | |||
| Iles and Sinclair | 1982 | Stable Retention Hypothesis | *Prey availability is not as critical to recruitment as the physical retention of the larvae, so spawning events must coincide with suitable wind and current conditions | *Adaptation of and forward progress on Hjort’s Aberrant Drift hypothesis |
| *Cites Atlantic Herring | ||||
| Sale | 1978; 1991 | Lottery Hypothesis | *Variability in tropical fish recruitment is dependent on pre- and post-settlement dynamics | *Contrasts Hjort’s original perspectives |
| *Supported by modern research (Doherty, 2002; Cowen 2002) | ||||
| *Cites tropical reef fish |
Case Study Evidence
To support this hypothesis, Reuben LaskerReuben lasker
Reuben Lasker was a fisheries scientist known for his contributions to larval ecology, particularly the Stable Ocean Hypothesis.-Early life and education:...
cited the disconnect between spawning stock biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
and the recruitment of numerous fish species. One explanation of this disconnect suggests that larval recruitment is influenced by spatial and temporal patterns of their food, like phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
or zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
, which can be greatly impacted by ocean currents and mixing. In his publication Marine fish larvae: Morphology, ecology, and relation to fisheries (1981), he points out, for example, the Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
vian anchovy
Anchovy
Anchovies are a family of small, common salt-water forage fish. There are 144 species in 17 genera, found in the Atlantic, Indian, and Pacific Oceans. Anchovies are usually classified as an oily fish.-Description:...
fishery collapse that resulted from a dramatic decrease in population size during the early 1970s. Officials and researchers from the Peruvian government and United Nations Food and Agriculture Organization submitted that the causal factors were a combination of strong fishing pressure and weak year classes that resulted in insufficient reproduction and recruitment to support the fishery
Fishery
Generally, a fishery is an entity engaged in raising or harvesting fish which is determined by some authority to be a fishery. According to the FAO, a fishery is typically defined in terms of the "people involved, species or type of fish, area of water or seabed, method of fishing, class of boats,...
. This explanation seemed to explain the diminished population trends of similar species from other regions, including the Pacific and Japanese sardine
Sardine
Sardines, or pilchards, are several types of small, oily fish related to herrings, family Clupeidae. Sardines are named after the Mediterranean island of Sardinia, around which they were once abundant....
s and the Atlanto-Scandian herring
Herring
Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...
.
Lasker, however, opposed this conclusion while citing the seemingly miraculous recovery of the troubled Japanese sardine population from scarcity (e.g. thousands of landed tons) to prominent abundance (e.g. more than a million landed tons). Another researcher studying the rebound of the Japanese sardine, Kondo (1980), identified an unusually strong 1972-year class, which produced successful recruitments in the years that followed. Kondo also noted altered ocean current patterns that increased zooplankton availability in spatio-temporal coincidence with the hatching of the sardine larvae. The result was increased larval survival and the eventual rebound of the population. Thus, the observed trend is that strong year class anomalies can have major impacts on population sizes and their future stability and growth. This concept also illustrates how plankton abundance and ocean currents can be driving factors associated with such trends. Clearly, these patterns become important when considering the predictive models necessary to manage and sustain important fisheries and the stocks that support them.
Experimental Support
Lasker also conducted studies using first-feeding anchovy larvae (during their critical period or the nutritional source transition from yolk sac to external food sources in very young larvae) to further test and eventually support his ideas. During one at-sea experiment, Lasker introduced water derived from various oceanic layers containing natural assortments of plankton as prey for the young anchovy larvae.He observed that anchovy larvae would first feed only when introduced to layers from below the surface containing high levels of chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
and certain species of phytoplankton. These feeding-suitable phytoplankton assemblages were correlated with non-mixing thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
layers. This conjecture was supported when foul weather swept up during the study and mixed the previously stratified layers (including a particulate layer below the surface) that had been sampled and introduced to the larvae. Samples derived from the same depths that proved ideal to induce first feeding prior to the storm no longer contained the necessary phytoplankton varieties and abundances; as a result, the young larvae did not feed on the post-storm samples and were unable to survive. Thus, Lasker eventually hypothesized that high-energy events that cause de-stratification of ocean layers have negative impacts on first-feeding northern anchovy larvae by destabilizing and decreasing their prey availability.
Conditions and Controversy of the Stable Ocean Hypothesis
In the book Advances in Marine Biology, Heath (1992) astutely outlines some of the limitations and controversyControversy
Controversy is a state of prolonged public dispute or debate, usually concerning a matter of opinion. The word was coined from the Latin controversia, as a composite of controversus – "turned in an opposite direction," from contra – "against" – and vertere – to turn, or versus , hence, "to turn...
associated with the SOH (Blaxter and Southward, 1992). Three major limiting conditions include, but are not limited to:
1) Species and region specific:
SOH is based primarily on experimental studies of Pacific anchovy in the California Current System
California Current
The California Current is a Pacific Ocean current that moves south along the western coast of North America, beginning off southern British Columbia, and ending off southern Baja California. There are five major coastal currents affiliated with upwelling zones...
(CCS) and should, therefore, be used cautiously when extrapolating to other species and regions (Heath, 1992).
2) Life stage specific:
Lasker’s suggestion that storms ultimately have negative impacts on first-feeding anchovy larvae should come with the disclaimer that this applies only to first- feeding larvae entering the critical period. Older anchovy larvae can, in fact, be positively affected by storm
Storm
A storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather...
activity, which often prompts increases in planktonic production (Heath, 1992).
3) Does not consider storm-absent survival of larvae:
Admittedly, one study supported the SOH by documenting a correlation between increased daily mortality rate
Mortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
s of first-feeding anchovy larvae and intervals of unperturbed, storm-absent conditions. Yet it is important to realize, as Peterman and Bradford did, that all this evidence does not affirm a certain relationship between larval survival and severe weather
Severe weather
Severe weather phenomena are weather conditions that are hazardous to human life and property.- Examples Include :Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, and , instability in the...
; this is particularly true given the lack of data regarding larval survival in the absence of storm events for context and comparison (Heath, 1992). Furthermore, Peterman and Bradford (1987) did not find a strong relationship between the larval survival rates documented adjacent to wind/storm events and recruitment levels observed months afterward.
Conclusions and Modern Perspective
Lasker’s Stable Ocean Hypothesis and the many other explanations of recruitment in larval variability are certainly relevant. However, current consensus is that larval recruitment and survival are, to some extent, dependent upon all of these processes and more; physical oceanography, prey, dispersal, and settlement/habitat are all very important factors, but predation, temperature, size, growth (Houde, 1989) etc. are other key aspects affecting recruitment. A lot of excellent research focusing on early life history and larval recruitment mechanisms was done from 1960–1990, and this body of research, including Lasker’s, laid the basis for larval ecology studies today. Presently, researchers around the world continue to study larval recruitment processes, but they still refer to the founding concepts that were written many years ago.Article written by Anne Dowling