Main Hypotheses of Georges Bank Study
Since the goal of the Georges Bank Study is to understand how physical
processes affect population dynamics, the general hypotheses of the
study reflect this focus. Based on the background material given above,
the major physical processes affecting abundance of the target species
lead to the following hypotheses:
LOCAL GROWTH vs. RETENTION/EXCHANGE
- Due to the circulation gyre, the residence time of water over the Bank is long relative to biological time scales so that in situ growth rather than lateral exchange is the dominant process controlling population abundance on the Bank.
- Fine-scale horizontal exchange causes significant leakage of nutrients, plankton, and fish larvae across the frontal boundaries of the Bank, thus causing a chronic input and exchange/loss of nutrients, plankton and fish larvae.
- Secondary circulation associated with the tidal mixing front causes a surface convergence near the well-mixed area boundary, providing a mechanism for concentrating target species in the tidal front zone. Transport towards the center of the Bank should be greatest for plankton in the upper layer of the water column in this zone, or for those species which undertake vertical migrations.
- Periodic vertical migration of zooplankton and juvenile fish into and out of the sheared bottom-boundary layer can lead to horizontal movement against the mean horizontal flow.
EPISODIC GAINS and EXCHANGES/LOSSES
- Seasonal density stratification over the southern flank of the Bank causes prey aggregation in the pycnocline and increased survival of predator populations.
- Differences in phytoplankton abundance and species composition mediated by differences in water column stability result in measureable differences in copepod recruitment and growth rates. This leads to greater abundances in one region over another, due solely to high growth rates in situ.
- Turbulent mixing, generated by wind and tidal forcing, has a significant impact on rates of ingestion, respiration, and predation; the processes of turbulent mixing and seasonal density stratification influence predator-prey encounter rates and thus growth and survival of individual organisms.
- The residual mean flow is important in horizontal transport of zooplankton and fish larvae onto and off of Georges Bank, thus causing major sources and sinks for Bank populations.
- The seeding of copepod populations from the Gulf of Maine during winter has a significant impact on the level of prey biomass for larval fish during late spring and early summer. A corollary is that the population genetic makeup of the prey on Georges Bank reflects the genetic makeup of the source populations.
- Storms, especially during winter and early spring, as well as impingement of warm-core rings, can cause large exchanges/losses of zooplankton and fish larvae from Georges Bank, thus increasing the apparent mortality rate of Bank populations.
- Population size is continuously regulated by incremental rather than episodic events, i.e., the time scale of the variability of the driving forces is of the same order as the generation time of the population.
These hypotheses provide the foundation from which the implementation
plan for the Georges Bank Study was developed, and upon which it will
continue to be developed. In the next section, program components, and
specific objectives and strategies for accomplishing them are presented.
- Predation rather than starvation is the dominant source of mortality of fish larvae; predation rather than advective exchange is the dominant source of mortality of copepods.