Chapter 10 Lipid Catabolism
10.1 Overview
Lipid catabolism provides a secondary energy pathway that fish use when immediate food resources are unavailable or when energy falls below a functional threshold. In this submodel, agents metabolize stored lipid reserves to restore energy, maintain migration, and avoid starvation, with lipid use influenced by body condition, metabolic efficiency, and the magnitude of energetic deficit. This process represents endogenous energy mobilization when metabolic demand exceeds energy intake, as energetic deficits generated by movement, digestion, and osmoregulation trigger the use of stored lipids to maintain function, consistent with established physiological responses in fishes during fasting, migration, and overwintering (Simpkins et al. 2003). In anadromous species, lipid reserves play a critical role in sustaining upstream migration and reproductive effort when feeding is reduced or suspended (Bordeleau et al. 2019).
Lipid depletion reduces body weight over time, creating a biologically meaningful cost to sustained reliance on internal reserves and linking energy balance to physiological state. Contaminant uptake does not occur during lipid catabolism, reflecting the absence of feeding. This formulation allows energy limitation and recovery to emerge dynamically from the interaction between metabolic demand and resource availability.
10.2 Purpose
The purpose of this submodel is to simulate endogenous energy mobilization through lipid reserves when fish cannot meet energetic needs through feeding. Lipid catabolism allows migratory fish to continue movement and survive temporary periods of low food availability. This mechanism reflects empirical observations of lipid use during spawning migrations, overwintering, and other energetically demanding life history periods.
This submodel is best used in tandpm with an energy recovery stratedge like filter feelding and digestion, or pref=dation and digesiton
10.3 Entities, State Variables, and Scales
10.3.1 Spatial and Temporal Scales
Spatial Unit: Patch (3 m x 3 m resolution)
Temporal Unit: 5 minute time steps (tick)
10.3.2 Global Variables
| Global Variable | Definition |
|---|---|
| full-energy-level | Target energy level used to compute energetic deficit during lipid use (default = 100). |
| energy-per-lipid | Energy returned per unit lipid mass mobilized (default = 40), used to convert lipid into energy. |
| lipid-refill-fraction | Fraction of body mass available to mobilize as lipid during fasting (default = 0.5). |
| lipid-max-fraction (optional) | Upper bound on lipid fraction that can be lost before mortality or physiological collapse (not currently enforced but biologically relevant). |
| debug-lipid? (optional) | Toggles printing of intermediate lipid catabolism calculations for debugging. |
10.3.3 Patch Variables
Lipid catabolism does not directly depend on patch level variables, since no external resources are consumed.
10.3.4 Agent Variables
| Variable Name | Definition |
|---|---|
| energy | Current energy level of the agent |
| weight | Current body mass of the fish |
| original-weight | Initial body mass used to scale maximum lipid availability |
| lipid-catabolism-efficiency | Efficiency with which lipids are converted into usable energy. This value is equal to the current metabolic rate divided by the metabolic rate at optimal temperature, allowing lipid use to scale with physiological performance |
| lipid-used | Amount of lipid converted to energy during the current step |
| metabolism-rate | Current metabolic rate, which influences efficiency scaling |
| baseline-metabolism-rate | Metabolic rate at optimal temperature |
| mehg-foraging | Methylmercury acquired during feeding (set to zero when lipids are used) |
| hg-foraging | Inorganic mercury acquired during feeding (set to zero when lipids are used) |
| mehg-foraging-total | Total methylmercury obtained from foraging |
| hg-foraging-total | Total inorganic mercury obtained from foraging |
| mehg-total | Total methylmercury accumulated |
| hg-total | Total inorganic mercury accumulated |
10.4 Process Overview and Scheduling
Compute energy deficit relative to a target energy level of one hundred (%, relative density).
Determine the lipid amount needed to fill the deficit (g tick⁻¹).
Limit lipid use by the maximum lipid available for mobilization (g tick⁻¹).
Apply lipid catabolism efficiency to determine usable lipid energy (%).
Convert mobilized lipid (g) to energy (energy units).
Reduce weight (g) by the amount of lipid burned (g).
10.5 Design Concepts
Basic Principles: Lipid catabolism represents endogenous energy mobilization when external food intake is insufficient. Fish draw upon stored lipids under energetic deficit, a well documented physiological response during migration, overwintering, and fasting (Simpkins et al. 2003). In anadromous species, these lipid reserves fuel upstream migration and reproductive effort when feeding is reduced or suspended [Bordeleau et al. (2019);].
Emergence: Patterns of weight loss, energy cycling, and recovery emerge from repeated lipid use during periods of limited food access.
Adaptation: Agents adaptively mobilize lipids based on the severity of their energy deficit and on efficiency scaling from the metabolism submodel.
Objectives: Agents seek to maintain enough energy to migrate, forage, avoid predation, and prevent starvation.
Sensing: Agents sense their own internal energy level and lipid reserve availability.
Interaction: Lipid catabolism does not require environmental interaction and therefore represents a purely endogenous process.
Stochasticity: This submodel is deterministic. However, variation in weight, metabolism, and previous feeding creates emergent differences among individuals.
Observation: Outputs include energy restored through lipid catabolism, lipid used, and body mass loss over time.
10.6 Initialization
| Variable | Initialized Value | Justification |
|---|---|---|
| weight | species specific | Establishes initial body condition |
| original-weight | equal to initial weight | Defines total lipid availability |
| energy | 100 | Full energy at initialization |
| lipid-catabolism-efficiency | from metabolism submodel | Reflects physiological scaling of lipid use |
| lipid-used | 0 | No lipid used at start |
10.7 Submodels
10.7.1 Lipid Mobilization
Maximum lipid available for mobilization depends on relative body mass:
\[ Lipid_{refill} = 0.5 \times \left(\frac{A_{mass}}{A_{mass_{i}}}\right) \hspace{2cm} \text{(10.1)} \]
Where:
- \(Lipid_{refill}\) is the Maximum lipid available for mobilization this tick (g).
- \(A_{mass}\) is the agent’s body mass used for allometric and energetic scaling (g).
Energy deficit relative to full energy:
\[ E_{deficit} = 100 - E_{agent} \hspace{2cm} \text{(10.2)} \]
Where:
- \(E_{deficit}\) is the energy shortfall relative to full energy (energy units).
- \(E_{agent}\) is the total internal energy available to the agent (energy units).
Lipid required to refill the deficit:
\[ Lipid_{needed} = \frac{Energy_{deficit}}{100} \hspace{2cm} \text{(10.3)} \]
Where:
- \(Lipid_{needed}\) is the lipid required to fill the energy deficit (g).
- \(E_{deficit}\) is the energy shortfall relative to full energy (%, relative energy).
Actual lipid mobilized:
\[ Lipid_{used} = \min(Lipid_{needed},\ Lipid_{refill}) \times Lipid_{Eff} \hspace{2cm} \text{(10.4)} \]
Where:
- \(Lipid_{used}\) is the lipid mobilized during the current tick (g).
- \(Lipid_{needed}\) is the lipid required to fill the energy deficit (g).
- \(Lipid_{refill}\) is the Maximum lipid available for mobilization this tick (g).
- \(Lipid_{Eff}\) is the efficiency multiplier applied to lipid catabolism (%).
10.7.2 Energy Conversion
Mobilized lipid is converted into energy:
\[ E_{added} = Lipid_{used} \times 100 \hspace{2cm} \text{(10.5)} \]
Where:
- \(E_{added}\) is the energy added from lipid mobilization (energy units).
- \(Lipid_{used}\) is the lipid mobilized during the current tick (g).
Energy is capped at one hundred:
\[ E_{agent} = \min(100,\ E_{agent} + E_{added}) \hspace{2cm} \text{(10.6)} \]
Where:
- \(E_{agent}\) is the total internal energy available to the agent (energy units).
- \(E_{added}\) is the energy added from lipid mobilization (energy units).
10.8 Netlogo Implementation
;; ===================================================================
;; LIPID CATABOLISM SUBMODEL DECLARATIONS
;; ===================================================================
turtles-own [
;; Energetics & body condition
energy
weight
original-weight
;; Lipid metabolism
lipid-catabolism-efficiency
lipid-used
;; Metabolic rate inputs
metabolism-rate
baseline-metabolism-rate
;; Contaminant bookkeeping during fasting
mehg-foraging
hg-foraging
mehg-foraging-total
hg-foraging-total
mehg-total
hg-total
]
;; ===================================================================
;; LIPID CATABOLISM SUBMODEL
;; ===================================================================
to lipid-loss
;; Maximum lipid metabolized this step, adjusted by efficiency
let lipid-refill 0.5 * (weight / original-weight)
;print (word "lipid-refill: " lipid-refill)
;print (word "current energy: " energy)
;; Calculate deficit and lipid need
let energy-deficit (100 - energy)
;print (word "energy-deficit: " energy-deficit)
let lipid-needed (energy-deficit / 40)
;print (word "lipid-needed: " lipid-needed)
;; Actual lipid use depends on efficiency and availability
let lipid-used (min list lipid-needed lipid-refill)
set lipid-used (lipid-used * lipid-catabolism-efficiency)
;print (word "lipid-used: " lipid-used)
;; Convert lipid to energy (scaled by efficiency)
let energy-change (lipid-used * 40)
;print (word "energy added: " energy-change)
set energy (energy + energy-change)
;print (word "energy after lipid conversion: " energy)
if energy > 100 [
set energy 100
print "energy capped at 100"
]
;; Weight loss proportional to lipid burned
set weight weight - (lipid-used)
;print (word "weight after lipid loss: " weight)
;; When relying on lipids, Hg mercury foraging risk stops
;; contaminant bookkeeping
let digested-MeHg 0
let digested-Hg 0
set mehg-foraging digested-MeHg
set hg-foraging digested-Hg
set mehg-foraging-total mehg-foraging-total + digested-MeHg
set hg-foraging-total hg-foraging-total + digested-Hg
set mehg-total mehg-total + digested-MeHg
set hg-total hg-total + digested-Hg
end