Fish Stock and Recruitment: Models, Density Dependence, and Population Dynamics

What is a biological stock?

A self-sustaining population of animals, not necessarily fixed geographically.

Define meta-population.

A group of sub-populations interconnected through the dispersal of at least some individuals.

What is a management unit in fisheries?

Fish contained within a convenient geographic region assumed to behave like a biological stock.

What does total stock biomass (TSB) refer to?

The weight of all fish in a stock at a certain time.

What is spawning stock biomass (SSB)?

The weight of mature fish in a stock at a certain time.

What does recruitment (R) mean in fisheries?

The number of individuals reaching a specified stage in their life cycle from the reproductive efforts of the parents during a certain time period.

What was Gulland's view on stock-recruit relations?

He suggested that the number of recruits is often independent of adult stock size over most observed ranges.

Why is the conclusion that recruitment is controlled by environmental conditions dangerous?

It implies that recruitment over-fishing is rare and stock collapses are solely due to poor environmental conditions.

Name four fish species linked to recruitment overfishing identified by Cushing.

Hokkaido-Sakhalin herring, Norwegian herring, Japanese sardine, California sardine.

What is the purpose of mathematical stock-recruit models?

To predict possible recruitment for different levels of SSB and recommend levels to avoid low recruitment.

What does a density-independent stock-recruit relationship imply?

The population stabilizes only if recruitment equals adult mortality; otherwise, it may explode or go extinct.

What is the significance of the 1:1 line in stock-recruit diagrams?

The population stabilizes at the point where the S-R curve crosses the 1:1 line.

What is a semelparous species?

A species that has only one spawning event in its lifetime.

What trend is observed in the S-R pattern for Skeena river sockeye salmon?

Larger spawning stocks tend to produce larger recruitments, but recruitment may plateau or decrease at high SSB.

What does the Beverton-Holt model describe?

A stock-recruit relationship suitable for stocks where recruitment tends to reach a fixed level.

What is the Ricker model used for?

A flexible model for stocks where recruitment decreases at high levels of SSB.

What are some potential mechanisms for density dependence during early life stages?

Over-saturation of spawning habitats, reduced growth rates increasing predation risk, and cannibalism.

What is the implication of the 0-0 point in stock-recruit curves?

Any chosen curve must pass through the origin, indicating no parents means no offspring.

What is the significance of recruitment variability in relation to spawning stock size?

Recruitment variability tends to be higher at larger spawning stock sizes.

How does the average S-R relationship behave with large scatter in data?

There are many potential curves that could fit the data, making it difficult to determine average trends.

What does the term 'over-compensation' refer to in the context of the Ricker model?

It allows for recruitment to exceed the number of spawners at low stock sizes.

What does the term 'density dependence' mean in fisheries?

It refers to processes that stabilize population sizes based on the density of individuals.

What is the role of environmental conditions in stock-recruitment relationships?

Environmental conditions can influence recruitment success, but their impact may vary across different stock sizes.

What is the consequence of an unbalanced recruitment and mortality rate?

The population may either explode or go extinct if the rates are not in equilibrium.

What is the main focus of the study by Andersen et al. (2017)?

The occurrence of density dependence in fish populations across different life stages.

What is the significance of Beverton and Iles' life-table for North Sea plaice?

It provides data on the survival rates of plaice, indicating that only 1 in every 40,000 eggs survives to its first birthday.

What does Ray Beverton's concentration hypothesis suggest?

Density-independent processes dominate in plankton, while density-dependent processes become significant when fish settle from a 3D habitat to a 2D nursery ground.

What challenge is associated with testing Beverton's concentration hypothesis?

Solid experimental evidence is hard to obtain due to the high costs of long-term field studies.

What does Ed Houde emphasize regarding mortality rates in early life stages of fish?

Tiny changes in mortality rates can lead to significant differences in recruitment across years.

What is the relationship between stock and recruitment in semelparous species?

Simple formulae exist to compute metrics like SMSY for these species.

What factors can lead to rapid increases in spawning stock biomass (SSB)?

Strong year-classes resulting from effective recruitment can lead to rapid increases in SSB, especially in heavily fished stocks.

What does the average stock-recruitment curve indicate about uncertainty?

There are wide uncertainty limits around average stock-recruitment curves due to variability in underlying data.

How can the uncertainty in stock-recruitment relationships be addressed?

By adding stochastic noise around the average stock-recruitment line using appropriate statistical distributions.

What was the status of the North Sea cod stock in the 1990s?

The stock was at a very low level, prompting management policies to control fishing mortality.

What did the 2003 model conclude about the North Sea cod stock recovery?

The stock could still be rebuilt despite global warming, but only for a limited time.

What recent trends have been observed in the North Sea cod stock since 2015?

The SSB has been declining rapidly, potentially due to increasing fishing mortality and poor recruitments.

What is the implication of depensation or Allee effects in fish stocks?

Stocks at low abundance may not recover effectively unless fishing pressure is significantly reduced.

What did Hilborn et al. (2014) conclude about fish populations at low abundance?

Most stocks are expected to rebuild when fishing pressure is reduced, but recovery time is uncertain and long.

What is a key takeaway regarding the variability in stock-recruitment relationships?

Empirical relationships show extreme annual variability, making predictions challenging.

What does the term 'stochasticity' refer to in the context of fish population models?

It refers to the incorporation of randomness in models to account for uncertainty in predictions.

What is the significance of the ICES reports mentioned in the notes?

They provide assessments of demersal stocks in the North Sea and Skagerrak, highlighting trends and management recommendations.

What can affect both egg numbers and quality in fish populations?

Factors such as female age and condition can impact egg production and quality, leading to variability in recruitment.

What does the term 'recruitment' refer to in fisheries science?

Recruitment refers to the addition of new individuals to a fish population, typically measured as the number of fish that survive to a certain age.

What is the role of climate change in fish population dynamics?

Climate change can affect recruitment and survival rates, potentially hindering stock recovery.

What does the term 'spawning stock biomass (SSB)' refer to?

SSB refers to the total weight of a fish population that is capable of reproduction.

Why is it important to consider uncertainties in fish population models?

Acknowledging uncertainties allows for more accurate predictions and better management strategies.

What does the term 'fishing mortality' mean?

Fishing mortality refers to the rate at which fish are removed from a population due to fishing activities.

What is a potential consequence of ignoring uncertainty in stock-recruitment models?

Ignoring uncertainty can lead to overly simplistic models that do not accurately reflect real-world dynamics.